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Journal of Speech, Language, and Hearing Research : JSLHR logoLink to Journal of Speech, Language, and Hearing Research : JSLHR
. 2020 Apr 13;63(4):1181–1194. doi: 10.1044/2019_JSLHR-19-00022

Comparing Tense and Agreement Productivity in Boys With Fragile X Syndrome, Children With Developmental Language Disorder, and Children With Typical Development

Elizabeth Hilvert a,✉, Jill Hoover b, Audra Sterling a,c, Susen Schroeder a
PMCID: PMC7242987  PMID: 32282263

Abstract

Purpose

This study compared and characterized the tense and agreement productivity of boys with fragile X syndrome (FXS), children with developmental language disorder (DLD), and children with typical development (TD) matched on mean length of utterance.

Method

Twenty-two boys with FXS (M age = 12.22 years), 19 children with DLD (M age = 4.81 years), and 20 children with TD (M age = 3.23 years) produced language samples that were coded for their productive use of five tense markers (i.e., third-person singular, past tense –ed, copula BE, auxiliary BE, and auxiliary DO) using the tense and agreement productivity score. Children also completed norm-referenced cognitive and linguistic assessments.

Results

Children with DLD generally used tense and agreement markers less productively than children with TD, particularly third-person singular and auxiliary BE. However, boys with FXS demonstrated a more complicated pattern of productivity, where they were similar to children with DLD and TD, depending on the tense marker examined. Results revealed that children with DLD and TD showed a specific developmental sequence of the individual tense markers that aligns with patterns documented by previous studies, whereas boys with FXS demonstrated a more even profile of productivity.

Conclusions

These findings help to further clarify areas of overlap and discrepancy in tense and agreement productivity among boys with FXS and children with DLD. Additional clinical implications of these results are discussed.

Strong language skills are necessary for communicating effectively with others and have downstream consequences for academic, cognitive, and social–emotional outcomes (e.g., Johnson et al., 2010). Yet, delays or impairments in language development are common among children with various neurodevelopmental disorders. As a result, there has been a burgeoning area of research aimed at identifying the unique language profiles of clinical groups. Empirical studies of this kind have significant clinical and theoretical implications in that they provide a greater understanding of the language processes that are impacted by disorders of known and unknown etiology. Despite varying underlying origins, studies of children with neurodevelopmental disorders such as developmental language disorder (DLD) 1 and fragile X syndrome (FXS) point to similar difficulties with grammar. In particular, the current study will focus on finiteness.

Finiteness is a dimension of grammar that is at the intersection of morphology and syntax as it involves marking tense and agreement on verbs (e.g., third-person singular –s, past tense –ed). Tense marking ability has long been considered a robust clinical marker of DLD (Rice & Wexler, 1996), and comparative studies have documented similar difficulties with tense marking in other concomitant disorders, including autism spectrum disorder (ASD) and Down syndrome (e.g., Eadie et al., 2002; Laws & Bishop, 2003; Tager-Flusberg, 2006). Recent evidence also points to deficits in tense marking for children with FXS (Estigarribia et al., 2011; Price et al., 2008; Sterling, 2018; Sterling et al., 2012), though it is less clear the extent to which precise patterns of tense marking may overlap with those seen in children with DLD (Haebig et al., 2016). Thus, the goal of this study was to gain a more comprehensive understanding of the similarities and differences in tense marking in children with FXS and DLD compared to a group of children with typical development (TD).

Tense Marking in Children With DLD and FXS

DLD is one of the most common pediatric language disorders (Norbury et al., 2016; J. B. Tomblin et al., 1997; also see Bishop et al., 2017, for a review). Prevalence estimates indicate that approximately 7% of U.S. children starting school present with DLD (J. B. Tomblin et al., 1997). Children with DLD demonstrate language deficits in the absence of hearing impairments, intellectual disability, or neurological impairments (Leonard, 2014). As mentioned above, difficulties mastering tense marking are well documented in young children with DLD, and these difficulties are typically greater than general delays in vocabulary and utterance length (i.e., measured via mean length of utterance [MLU]; e.g., Rice et al., 2009; Rice & Wexler, 1996, Rice et al., 1998). More specifically, children with DLD frequently omit tense markers in everyday speech (e.g., she run) and show delays in the use or mastery of these morphemes in experimental production tasks (Rice & Wexler, 1996, 2001; Rice et al., 1998). The omission of tense markers is persistent in DLD. For example, in a series of studies, Rice and Wexler (1996, 2001) found that less than 10% of 5-year-olds with DLD had mastered tense marking compared to at least 80% of 5-year-olds with TD.

Although delays in the mastery of tense marking are considered a hallmark feature of DLD, they are not limited to children with DLD. This has motivated comparative studies between DLD and other neurodevelopmental disorders that have noted areas of commonality as well as some distinctions in tense development (Eadie et al., 2002; Haebig et al., 2016; Laws & Bishop, 2003; J. A. Roberts et al., 2004). These findings have contributed to the ongoing debate as to whether similar developmental processes may be perturbed in these disorders that lead to overlapping language difficulties (Ellis Weismer, 2013; B. Tomblin, 2011). Among these comparison studies, one disorder that has received limited attention is FXS, despite children also presenting with weaknesses in finiteness. Therefore, examining whether children with FXS may demonstrate a similar or distinct tense marking profile compared to children with DLD is important for shedding further light on this debate.

FXS is the leading inherited cause of intellectual disability, which affects around one in 2,500–4,000 boys and one in 8,000 girls (Crawford et al., 2001; Fernandez-Carvajal et al., 2009). It is the result of a single gene (FMR1) mutation on the X chromosome (Verkerk et al., 1991), which disrupts the production of a protein that is necessary for normal brain development. As a result, individuals with FXS often present with language impairments, hyperactivity, and social anxiety, in addition to an intellectual disability (Cordeiro et al., 2011; Sterling et al., 2012; Sullivan et al., 2006). A substantial number of individuals with FXS also demonstrate behaviors similar to those with ASD, with approximately 27%–75% receiving a co-diagnosis of ASD (Clifford et al., 2007; Klusek et al., 2014). Given the X-linked nature of FXS, not only are diagnoses of ASD more common in males, but males are generally more severely affected than females (Hagerman & Hagerman, 2002). Thus, in this study, we focused on boys with FXS in order to control for sex differences commonly observed for children with FXS.

A number of studies have indicated that boys with FXS demonstrate delays in grammar, beyond what would be expected given their nonverbal mental age (e.g., Estigarribia et al., 2011; Finestack et al., 2013; Martin et al., 2013; Price et al., 2008). More specifically, boys with FXS demonstrate significantly shorter conversational MLU compared to children with TD matched on nonverbal mental age (e.g., Finestack et al., 2013; Kover et al., 2012). Others have also found that nonverbal mental age–matched boys with TD outperform boys with FXS in their use of noun phrases, verb phrases, and sentence structure (Price et al., 2008; J. E. Roberts et al., 2007). Although limited, a growing body of research has begun to specifically examine the tense marking abilities of boys with FXS. For example, Estigarribia et al. (2011) found that boys with FXS produced fewer total morphosyntactic verbs that marked tense in conversational language samples than boys with TD, after controlling for nonverbal mental age, maternal education, and articulation.

Sterling and colleagues (Haebig et al., 2016; Sterling, 2018; Sterling et al., 2012) extended this work by examining tense marking at a more detailed level, evaluating performance on specific types of tense markers using the Test of Early Grammatical Impairment (TEGI; Rice & Wexler, 2001). The TEGI is a norm-referenced assessment that is designed to elicit tense markers in an obligatory context and has probes that assess third-person singular, past tense (regular and irregular), as well as copula and auxiliary BE and auxiliary DO verbs. Sterling (2018) compared the tense marking abilities of boys with FXS to boys with idiopathic ASD who were similar in terms of ASD severity and MLU. Both groups of boys demonstrated a strength in their accurate use of third-person singular and copula and auxiliary BE, but they had a relative weakness with regular past tense and auxiliary DO morphemes. This pattern is interesting considering the typical developmental course or emergence of these morphemes (Stage III: copula BE; Stage IV: past tense –ed and third-person singular; and Stage V: auxiliary BE and DO; Brown, 1973). In any case, boys with FXS demonstrated greater difficulty producing auxiliary BE verbs on the TEGI than boys with idiopathic ASD. This is noteworthy as research has indicated that at least a subset of children with ASD has deficits in the use of tense and agreement marking (J. A. Roberts et al., 2004; Tager-Flusberg, 2006). Sterling and colleagues also evaluated the tense marking abilities of boys with FXS in relation to other aspects of their language and found that tense marking performance was below receptive vocabulary expectations specifically for third-person singular and past tense morphemes (Sterling et al., 2012). However, their overall tense marking performance exceeded expectations benchmarked to MLU. This latter finding suggests that boys with FXS may demonstrate a unique profile compared to children with DLD, as tense marking ability is disproportionally poorer than general delays in MLU in children with DLD (e.g., Hoover et al., 2012; Leonard et al., 1992).

However, to our knowledge, only one study has directly compared the grammatical abilities of boys with FXS to children with DLD. Using the TEGI, Haebig et al. (2016) found, as expected, that comparisons to the normative scores in the TEGI manual indicated that the boys with FXS generally performed below nonverbal mental age expectations (Rice et al., 2010). However, the boys with FXS were significantly better on all tense marker probes compared to younger, MLU-matched children with DLD. In line with the results of Sterling et al. (2012), boys with FXS instead performed similarly to MLU-matched children with TD on the TEGI and even outperformed the children with TD on third-person singular productions. Together, these studies provide an initial profile of tense marking ability in FXS—that is, tense marking abilities appear to be in line with MLU expectations and exceed the performance of younger children with DLD, but these previous studies have primarily relied on information from norm-referenced tests. Extending this comparison between children with FXS and DLD using other types of assessment, particularly those derived from spontaneous language, is essential in order to explore the differentiation between the grammatical phenotypes of these clinical groups.

Methods for Assessing Tense Marking

It is important to consider the type of language assessments that are being utilized as varying techniques have different intrinsic properties and tap into different dimensions of the same construct (e.g., tense marking accuracy vs. production). Understanding these subtle nuances can provide a deeper understanding of language performance and the degree to which it appears to be impaired (e.g., Kover et al., 2012). With regard to tense marking, assessment methods generally fall into two categories: (a) those that explicitly target the comprehension or production of tense markers through either experimentally designed probes or norm-referenced measures and (b) those that assess spontaneous tense marking using language samples. The TEGI is one example of a norm-referenced assessment. Children's performance on tense marker probes is measured by accuracy, which can be compared to a normative sample. Because this has been previously used to evaluate tense marking in FXS, we were interested in using the latter method—examining spontaneous production of tense marking in a language sample. The aim of doing so was to gain a deeper understanding of tense marking, beyond accuracy, by examining tense production in boys with FXS and how it compares to children with DLD and TD.

When evaluating tense marking using language samples, researchers will often use composite scores that examine the productive use of tense and agreement. These include measures such as the finite verb morphology composite (Leonard et al., 1999), the tense composite (Rice et al., 1998), and the tense and agreement productivity (TAP) score (Hadley & Short, 2005). The finite verb morphology composite and the tense composite are similar in that they both provide a combined percentage of tense/agreement morphemes used in obligatory contexts. Although both scores provide valuable information, one problem that has been identified with using these composites is that children receive credit for each use of the morphemes of interest, including tense markers that appear in high-frequency and potentially memorized morpheme combinations (e.g., it's, that's). As such, these composite scores may overestimate a child's tense marking abilities.

For this reason, we used the TAP score in this study. The TAP score was designed to examine tense marking in “diverse, low frequency sentences frames” as a way to provide an accurate estimate of the productivity or depth of the tense and agreement system early on in development (Rispoli & Hadley, 2018, p. 1457). It is computed by identifying children's productive use of (a) third-person singular (e.g., he walks), (b) regular past tense –ed (e.g., she flipped), (c) copula BE (e.g., the boy is sleepy), (d) auxiliary BE (e.g., he is getting out), and (e) auxiliary DO (e.g., do they eat carrots?). However, there are specific productivity criteria to protect against artificially inflated scores. Children are only able to receive a maximum of 5 points for each tense marker when using it in up to five sufficiently different syntactic contexts. To be considered sufficiently different, third-person singular and past tense –ed forms must occur on different verbs (e.g., wanted, looked), whereas copula BE, auxiliary BE, and auxiliary DO must occur with different sentence subjects (e.g., The boy is sleepy; The girl is at the mall). Moreover, to further reduce the chance of overestimating tense mastery, all instances in which contracted copula or auxiliary BE appears with pronoun subjects (e.g., it's, that's) are excluded.

In their initial study, Hadley and Short (2005) were able to differentiate 2-year-old children at risk for DLD from children with low-average language abilities using the TAP score. More recently, research has shown that the TAP score is also able to distinguish the tense marking abilities of older, preschool-age children with DLD and age-matched children with TD (Gladfelter & Leonard, 2013; Guo & Eisenberg, 2014). Hadley and Short found that copula BE and third-person singular were the first forms to emerge and were most consistently productive. Finally, the TAP score is highly correlated with other measures of expressive vocabulary and grammar taken from language samples (i.e., number of different words [NDW], MLU, Index of Productive Syntax; Hadley & Short, 2005).

Overview of This Study

The current study extends previous research by comparing and characterizing the tense and agreement productivity of boys with FXS and MLU-matched children with DLD and TD using spontaneous language samples. We were interested not only in examining differences between these three groups in terms of productivity as measured by the TAP score but also in examining the pattern of productivity across individual tense and agreement morphemes within each clinical group. This was motivated by the fact that there is still limited understanding of the pattern of tense and agreement emergence in children with FXS in particular, compared to what we know about children with TD and DLD. Moreover, though the TAP score is related to other conversational measures of expressive grammar (Hadley & Short, 2005), research is needed to understand how the TAP score may be correlated to norm-referenced measures of tense marking, such as the TEGI. We were also interested in examining the association between tense productivity and lexical diversity (i.e., NDW) in order to explore whether children's expressive grammar ability aligned with their expressive vocabulary ability. Thus, our research questions were the following: (a) Do boys with FXS, children with DLD, and children with TD all matched on MLU differ in their tense and agreement productivity (i.e., total productivity, productivity of specific tense markers)? (b) Are children using certain types of tense markers more productively than others, and does this pattern differ across the three groups? (c) Is the tense and agreement productivity of children with FXS, DLD, and TD related to their performance on other measures of expressive language (i.e., NDW, MLU, and the TEGI)?

Based on previous findings (Haebig et al., 2016; Sterling et al., 2012), we expected that the boys with FXS would produce more sufficiently different tense and agreement markers than MLU-matched children with DLD, and instead look similar to MLU-matched children with TD. We also expected that boys with FXS would demonstrate a pattern of productivity that more closely aligned with that of children with TD than that of children with DLD. However, it is possible that our findings may not simply mirror those of previous studies given that a different assessment technique is being used in the current study (language sampling vs. the TEGI). In particular, language sampling draws more heavily on pragmatics, which may present a particular challenge for the boys with FXS. Thus, we alternatively hypothesized that the boys with FXS could look more similar to the MLU-matched children with DLD than the children with TD when assessing tense marking via language sampling. In support of this, Finestack et al. (2013) found that boys with FXS did not differ from nonverbal mental age–matched peers with TD on a standardized measure of expressive grammar, but the boys with FXS did have lower MLUs. Finally, we predicted that expressive vocabulary (NDW) and grammar (MLU, TEGI) would be associated with children's TAP scores, with a similar pattern of associations found across the three groups.

Method

Participants

The study included 61 children: 22 with FXS, 19 with DLD, and 20 with TD. The children with FXS were a subsample of participants from a larger study examining grammar and language assessment (Friedman et al., 2018; Haebig & Sterling, 2017; Haebig et al., 2016; Sterling, 2018). The children with TD and DLD participated in a larger study focused on identifying factors influencing the omission of finiteness markers (Hoover et al., 2012). All children were monolingual speakers of Standard American English. All parents provided written informed consent, and children provided assent. Study procedures were approved by the institutional review boards.

The children with FXS were all boys who had the full mutation, confirmed via previous genetic testing. All boys with FXS met ASD criteria on both the Autism Diagnostic Observation Schedule (or Autism Diagnostic Observation Schedule–Second Edition; Lord et al., 1999, 2012) and the Autism Diagnostic Interview–Revised (Rutter et al., 2003), a caregiver-based interview of children's past and present behaviors. The children with DLD included 12 boys and seven girls. The presence of DLD was confirmed either by an existing diagnosis of language impairment by a speech-language pathologist or by performance below age expectations on MLU and the elicited grammar composite of the TEGI (Rice & Wexler, 2001). All children with DLD had normal intellectual abilities, confirmed by scores on a test of nonverbal IQ (Reynolds Intellectual Assessment Scales [RIAS]; Reynolds & Kamphaus, 2003), and normal hearing ability. Though all of the children with DLD performed below age expectations for MLU and the TEGI, only some also had significant weaknesses in vocabulary (see receptive vocabulary scores in Table 1). Children with TD included eight boys and 12 girls that scored within the normal range on the complete diagnostic testing battery (TEGI, MLU, Peabody Picture Vocabulary Test–Fourth Edition, and RIAS) and had normal hearing ability.

Table 1.

Participant characteristics.

Variable FXS (n = 22) DLD (n = 19) TD (n = 20)
Chronological age 12.22 (1.98) 4.81 (0.65) 3.23 (0.26)
Nonverbal cognition (standard score) a 47.68 (8.48) 111.68 (16.20) 116.47 (12.65)
Nonverbal mental age equivalence a 5.32 (0.65) 5.27 (1.10) 4.14 (0.59)
MLU in morphemes 3.79 (1.03) 3.99 (0.73) 4.08 (0.77)
NDW 128.68 (24.62) 111.32 (19.89) 113.90 (20.21)
Racial identity
 White 91.0% 73.7% 95.0%
 African American 0.0% 15.8% 5.0%
 Other 4.5% 0.0% 0.0%
 More than one race 4.5% 10.5% 0.0%
Ethnic identity
 Not Hispanic/Latino 85.7% 94.7% 100%
 Hispanic/Latino 14.3% 5.3% 0.0%
TEGI elicited grammar composite b 70.68 (16.82) 29.00 (14.80) 61.63 (8.90)
Receptive vocabulary (standard score) c 61.45 (14.72) 95.84 (11.51) 112.70 (10.55)
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Note. FXS = fragile X syndrome; DLD = developmental language disorder; TD = typical development; MLU = mean length of utterance; NDW = number of different words.

a

Leiter International Performance Scale–Revised (Roid & Miller, 1997) or Reynolds Intellectual Assessment Scales (Reynolds & Kamphaus, 2003).

b

Test of Early Grammatical Impairment (TEGI; Rice & Wexler, 2001).

c

Peabody Picture Vocabulary Test–Fourth Edition (Dunn & Dunn, 2007).

Group Comparisons

Participants with FXS, DLD, and TD were well matched on MLU, F(2, 58) = 0.64, p = .529, ηp 2 = .02, following the conventions set forth in Kover and Atwood (2013). MLU-based comparisons are common in studies of children with neurodevelopmental disorders (e.g., Haebig et al., 2016; Levy et al., 2006; Rice et al., 1998; Sterling, 2018). MLU is a robust index of language acquisition. Moreover, MLU-based comparisons help control for important confounds (e.g., utterance length) and allow for findings to be evaluated within the broader literature on tense marking.

In terms of nonverbal mental age, overall, the groups differed significantly, F(2, 58) = 14.00, p = .0001, ηp 2 = .33. Follow-up independent-samples t tests with Holm–Bonferroni corrections revealed that children with TD had a younger nonverbal mental age equivalence compared to children with DLD, p = .0003, and boys with FXS, p = .0003. However, when comparing the two clinical groups, the children with DLD and boys with FXS were well matched, p = .866. This difference in nonverbal mental age equivalence between children with DLD and TD is not surprising, as these groups were initially matched on MLU, and to do so, the children with TD were chronologically younger. As expected, there was also a significant difference in children's nonverbal IQ standard scores, F(2, 58) = 192.77, p = .0001, ηp 2 = .87, with boys with FXS having significantly lower scores compared to children with TD, p = .0003, and children with DLD, p = .0003. Importantly, the children with DLD and TD were similar in terms of their nonverbal IQ standard scores, p = .316. See Table 1 for additional information.

Materials

Nonverbal IQ

Two different assessments of nonverbal cognition were used. The children with FXS completed the Leiter International Performance Scale–Revised (Roid & Miller, 1997). The children with DLD and the children with TD completed the two subtests from the Nonverbal Intelligence Index of the RIAS (Reynolds & Kamphaus, 2003). Standard scores and mental age equivalence scores were calculated for each assessment.

Language Assessments

The Peabody Picture Vocabulary Test–Fourth Edition (Dunn & Dunn, 2007) is a norm-referenced assessment of vocabulary comprehension that was used to describe the participants. The TEGI (Rice & Wexler, 2001) was administered to assess children's expressive grammatical abilities. The TEGI is a clinical tool that is used for the screening, identification, and diagnosis of grammatical impairments in children between 3 and 8 years old. The TEGI specifically examines finite verb morphology. Although the TEGI has been used primarily to evaluate language abilities in children with DLD (e.g., Hoover et al., 2012; Rice & Wexler, 2001), it has also been used with children with FXS (Haebig et al., 2016; Sterling, 2018; Sterling et al., 2012). Prior to evaluating grammar ability, the phonological probe of the TEGI was administered to ensure that children were able to correctly produce the phonemes that are required to produce finiteness markers (i.e., /s/, /z/, /t/, /d/) in monomorphemic words (e.g., “bus,” “cheese,” “hat,” “bed”). All children passed the phonological probe by demonstrating accurate production of the relevant phonemes and were subsequently administered the three probes that comprise the elicited grammar composite: third-person singular, past tense (regular and irregular), and BE/DO (copula BE and auxiliary BE and DO). For these subtests, a prompt is provided that elicits a sentence containing an obligatory context for each given tense marker. Responses are scored as correct, incorrect, or unscorable. Responses are incorrect if the child omits the finiteness marker. A response is unscorable when a child produces a nontarget tense (e.g., past tense during third-person singular probe) or a nontarget subject (e.g., plural subject in third-person singular probe) within a particular subtest. The TEGI does not include unscorable responses when calculating children's accuracy. Therefore, the elicited grammar composite reflects the percentage of correct responses out of total scorable responses.

Language Samples

Prior research suggests that conversational sampling during play-based activities is the most appropriate method for eliciting a language sample from young children, whereas a semistructured interview is considered to be more appropriate for eliciting a conversational language sample from older, school-age children (e.g., Evans & Craig, 1992; Haebig et al., 2016; Miller, 1981). Moreover, studies comparing language sampling contexts suggest that more complex grammar may be elicited from less structured, play-based situations in young children, whereas more complex grammar is elicited from interview-style conversations in school-age children (Evans & Craig, 1992; Longhurst & File, 1977; Southwood & Russell, 2004; Westerveld et al., 2004). Thus, in line with prior research (Haebig et al., 2016), we selected sampling procedures that were considered age appropriate for each group considering both chronological and mental age in order to elicit comparable MLU—boys with FXS completed a 10-min conversation-based language sample, and the children with DLD and TD completed a 30-min play-based language sample. In the semistructured conversation sample, we followed the procedures developed by Berry-Kravis et al. (2013), which were created for obtaining language samples from children with FXS. The examiner followed a list of conversation topics, starting with the participant's specific interest and moving to a set list of topics (e.g., sports, pets, school). The examiner used elicitation techniques (e.g., open-ended questions) to encourage the child to talk, while at the same time minimizing their own talk (Berry-Kravis et al., 2013). During the play-based sample, the examiner and child played with age-appropriate toys, and the examiner engaged in toy talk with the child, which, for younger children, is an effective method of eliciting utterances that should be marked for tense and agreement (Hadley & Walsh, 2014).

Children's language samples were transcribed using the Systematic Analysis of Language Transcripts (SALT) procedures (Miller et al., 2011) by trained and reliable undergraduate and graduate student coders. Utterances were segmented according to C-units, that is, an independent clause and all attached subordinate clauses. MLU in morphemes and NDW were generated using SALT and were based on complete and intelligible utterances.

TAP Scoring

Children's language transcripts were coded for their use of tense markers using the TAP score (Hadley & Short, 2005). The TAP score is computed by identifying children's productive use of (a) third-person singular present tense, (b) regular past tense –ed, (c) copula BE, (d) auxiliary BE, and (e) auxiliary DO. Children earned a score between 0 and 5 for each morpheme, with 1 point awarded for each sufficiently different use of the morpheme. Across the full set of morphemes, the total tense productivity score ranged from 0 to 25, with higher scores indicating higher levels of tense and agreement morpheme productivity in unique syntactic contexts. To be considered a sufficiently different use, correct uses of third-person singular and regular past tense –ed had to occur with different lexical verbs (e.g., She needed help; The dog dropped the ball). For the regular past tense –ed score, overregularization of the verb inflection (e.g., goed) was counted as correct, but all other errors were not considered correct (e.g., the children plays with the ball). Correct productions of copula BE, auxiliary BE, and auxiliary DO were counted in the score as long as they occurred with different sentence subjects (e.g., the boy is small; the girl is loud). For copula and auxiliary BE, all sentences that had contracted pronominal subjects (e.g., she's finished) or wh-words (e.g., what's that sound?) were excluded to avoid the possibility of overcrediting potentially unanalyzed, lexically specific forms. For specific examples of TAP coding, see the Appendix.

Given that the duration of language sampling procedures differed for boys with FXS and children with DLD and TD, for the purpose of calculating the TAP score, as well as MLU and NDW, the first 100 utterances were selected from children's language samples. This allowed us to equate the sample size from which tense markers were obtained. Additionally, the first 100 utterances were selected because research has shown that TAP scoring on 100 utterance samples has better diagnostic accuracy than scores from 50 utterance samples (Guo & Eisenberg, 2014).

Reliability

SALT Reliability

For reliability coding, we randomly selected five boys with FXS (23%), four children with DLD (21%), and four children with TD (20%) and had a second trained coder independently transcribe these language samples in SALT. Interobserver word agreement was 83% for boys with FXS, 90% for the children with DLD, and 90% for the children with TD. Interobserver agreement for the presence or absence of bound morphemes was 81% for the FXS group, 89% for the DLD group, and 89% for the TD group.

TAP Reliability

A second independent coder completed TAP score coding for five boys with FXS (23%), four children with DLD (21%), and four children with TD (20%). When examining productive and unproductive uses of tense and agreement, interobserver agreement was 94% for past tense –ed, 97% for third-person singular, 89% for copula BE, 97% for auxiliary BE, and 92% for auxiliary DO.

Data Analysis

Question 1

To address our first question, we ran a one-way analysis of variance (ANOVA) to explore group differences on the TAP total score and a multivariate analysis of variance (MANOVA) to assess group differences on the five individual tense markers, that is, third-person singular, past tense -ed, copula BE, auxiliary BE, and auxiliary DO. In both analyses, diagnostic group was the fixed factor. Main effects of diagnostic group were followed up with planned t tests, using a Holm–Bonferroni correction to adjust for multiple comparisons (Holm, 1979).

Because there was a significant difference in nonverbal IQ between boys with FXS and children with DLD and TD, and a significant difference in nonverbal mental age between children with TD and children with DLD and FXS, both variables were initially considered as covariates. Correlation analyses were completed to examine the relation between nonverbal IQ, nonverbal mental age, and the dependent variables. The analyses revealed that both nonverbal IQ (standard score) and nonverbal mental age were not correlated with any of the dependent variables for boys with FXS, r(20) ≤ .35, ps ≥ .140; children with DLD, r(17) ≤ .37, ps ≥ .125; and children with TD, r(18) ≤ −.35, ps ≥ .142. Given the lack of significant correlations, we chose not to include these factors as covariates. Additionally, a number of methodological and theoretical concerns regarding the use of nonverbal IQ as a covariate have been outlined by Dennis et al. (2009). Specifically, adjusting for nonverbal IQ in clinical populations such as FXS makes interpretation of results difficult, given that intellectual disability is a central part of the disorder (Dennis et al., 2009). Similarly, despite boys with FXS being older than the children with DLD and TD, chronological age was also not related to any of the dependent variables in boys with FXS, r(20) ≤ .32, ps ≥ .150, and children with TD, r(18) ≤ −.32, ps ≥ .171, and only related to the production of copula BE in children with DLD, r(17) = .49, p = .033. Thus, we chose not to control for chronological age.

Question 2

Research question two examined the pattern of tense and agreement productivity within each group. We completed separate repeated-measures ANOVAs for each diagnostic group. The within-subject variable was tense type (third-person singular, past tense –ed, copula BE, auxiliary BE, auxiliary DO). Main effects of tense type were followed up with t tests, using a Holm–Bonferroni correction to adjust for multiple comparisons (Holm, 1979).

Question 3

For our third research question, Pearson correlations were completed to determine whether children's TAP total scores were related to NDW, MLU, and the elicited grammar composite score on the TEGI. Separate correlations were run for each diagnostic group.

Results

Question 1: Differences Between FXS, DLD, and TD in TAP Performance

ANOVA analyses revealed a significant effect of diagnostic group for the TAP total score, F(2, 58) = 5.52, p = .006, ηp 2 = .16 (see Table 2). Follow-up independent-samples t tests with Holm–Bonferroni corrections showed that children with DLD scored lower on their overall TAP performance than children with TD, t(37) = 3.79, p = .003, but their performance did not differ from boys with FXS, t(39) = 1.85, p = .140. Boys with FXS also did not differ from children with TD, t(40) = 1.35, p = .184. When examining the use of individual tense markers, the MANOVA revealed a significant main effect of diagnostic group on tense production, F(10, 110) = 3.88, p = .0001, ηp 2 = .26. Follow-up ANOVAs revealed a significant main effect of diagnostic group for the use of third-person singular, F(2, 58) = 6.36, p = .003, ηp 2 = .18; auxiliary BE, F(2, 58) = 5.07, p = .009, ηp 2 = .15; and auxiliary DO, F(2, 58) = 4.60, p = .014, ηp 2 = .14. No main effect of diagnostic group was found for copula BE, F(2, 58) = .16, p = .855, ηp 2 = .01, or past tense –ed, F(2, 58) = 1.56, p = .219, ηp 2 = .05.

Table 2.

Comparison of tense and agreement productivity scores across groups.

Variable FXS DLD TD
Copula BE 3.23 (1.63) 3.00 (1.49) 3.00 (1.41)
Third-person singular 1.95 (1.81) b 2.11 (1.33) c 3.55 (1.50) b , c
Past tense –ed 1.23 (1.41) 0.58 (1.07) 1.05 (1.05)
Auxiliary BE 0.91 (0.97) 0.68 (0.89) c 1.75 (1.41) c
Auxiliary DO 1.41 (1.44) a 0.42 (0.77) a 0.70 (0.87)
Total score 8.77 (3.73) 6.79 (3.01) c 10.10 (2.43) c
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Note. Standard deviation in parentheses. FXS = fragile X syndrome; DLD = developmental language disorder; TD = typical development.

a

Significant difference between FXS and DLD.

b

Significant difference between FXS and TD.

c

Significant difference between DLD and TD.

For third-person singular, follow-up analyses revealed that children with DLD, t(37) = 3.17, p = .003, and boys with FXS, t(40) = 3.09, p = .009, produced significantly fewer unique instances of the morpheme compared to children with TD. However, boys with FXS and children with DLD did not differ from one another in their production of third-person singular, t(39) = 0.29, p = .766. Similarly, for auxiliary BE, children with DLD produced fewer different instances of auxiliary BE compared to children with TD, t(37) = 2.81, p = .008. The clinical groups (FXS and DLD) did not differ from one another in their use of auxiliary BE, t(39) = −0.77, p = .446. With regard to auxiliary DO, children with DLD produced fewer instances of this tense marker compared to boys with FXS, t(33.00) = −2.80, p = .027, but did not differ from children with TD, t(36.85) = 1.07, p = .293. Moreover, boys with FXS did not differ from children with TD in their use of auxiliary BE, t(40) = 2.27, p = .058, or auxiliary DO, t(34.94) = −1.96, p = .116. See Table 2 for means and standard deviations for each group.

Question 2: Patterns of Tense Productivity in Children With FXS, DLD, and TD

Tense Marking in Boys With FXS

The repeated-measures ANOVA revealed a significant main effect of tense marker type for boys with FXS, F(4, 84) = 8.95, p = .0001, ηp 2 = .30. Follow-up paired-samples t tests with Holm–Bonferroni corrections revealed children were more productive with their use of copula BE than past tense –ed, t(21) = 4.87, p = .001; auxiliary BE, t(21) = 6.39, p = .001; and auxiliary DO, t(21) = 4.57, p = .001. However, the productivity of copula BE did differ from third-person singular, t(21) = 2.45, p = .161. Moreover, no differences in the productivity or use of the other four tense markers (i.e., third-person singular, past tense –ed, auxiliary BE, and auxiliary DO) were found, ts(21) ≤ 2.06, ps ≥ .312 (see Figure 1).

Figure 1.

Figure 1.

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Pattern of tense and agreement productivity across groups. The solid line indicates the median, and the cross indicates the mean. FXS = fragile X syndrome; DLD = developmental language disorder; TD = typical development.

Tense Marking in Children With DLD

Analyses revealed a significant main effect of tense marker type for children with DLD, F(4, 72) = 21.01, p = .0001, ηp 2 = .56. Paired-samples t tests with Holm–Bonferroni corrections revealed that children were more productive in their use of copula BE than past tense –ed, t(18) = 5.85, p = .0009; auxiliary BE, t(18) = 7.33, p = .0009; and auxiliary DO, t(18) = 4.59, p = .0009. However, the use of copula BE and third-person singular did not differ, t(18) = 2.22, p = .160. Moreover, children with DLD used third-person singular more productively than past tense –ed, t(18) = 3.75, p = .005; auxiliary BE, t(18) = 4.47, p = .001; and auxiliary DO, t(18) = 4.59, p = .001. No differences were found though between children's use of past tense –ed, auxiliary BE, and auxiliary DO, ts(18) ≤ −1.16, ps ≥ .786 (see Figure 1).

Tense Marking in Children With TD

A main effect of tense marker type was also found for children with TD, F(4, 76) = 21.01, p = .0001, ηp 2 = .54. Follow-up paired-samples t tests with Holm–Bonferroni corrections revealed that children with TD were significantly more productive with copula BE than past tense –ed, t(19) = 6.35, p = .001, and auxiliary DO, t(19) = 6.19, p = .001. Children with TD also used third-person singular more productively than past tense –ed, t(19) = 6.35, p = .001; auxiliary BE, t(19) = 3.39, p = .018; and auxiliary DO, t(19) = 6.19, p = .001. Finally, children with TD were more productive with auxiliary BE than auxiliary DO, t(19) = −3.28, p = .020. No other significant differences between tense markers were found for children with TD, ts(19) ≤ 2.70, ps ≥ .056 (see Figure 1).

Question 3: Associations Between the TAP Score and Measures of Expressive Vocabulary and Grammar

As shown in Table 3, Pearson correlation analyses revealed that TAP total scores were positively correlated to NDW in boys with FXS and children with TD, but not children with DLD. With regard to MLU, boys with FXS, children with DLD, and children with TD with higher TAP total scores had longer MLUs. However, regardless of group, no significant associations were found between the TAP total score and the TEGI composite score (see Table 3).

Table 3.

Correlations between tense and agreement productivity (TAP) total score and measures of expressive vocabulary and grammar.

Variable Boys with FXS
TAP total score
r
p
NDW .70 .0001
MLU .70 .0001
TEGI elicited grammar composite
 .30
 .172
Children with DLD
TAP total score
r
p
NDW .26 .279
MLU .46 .048
TEGI elicited grammar composite
 .26
 .278
Children with TD
TAP total score
r
p
NDW .46 .043
MLU .74 .0001
TEGI elicited grammar composite −.071 .766
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Note. FXS = fragile X syndrome; NDW = number of different words; MLU = mean length of utterance; TEGI = Test of Early Grammatical Impairment; DLD = developmental language disorder; TD = typical development.

Discussion

The grammatical system plays a fundamental role in the ability to communicate effectively with others and in the development of academic skills such as reading and writing. For this reason, deficits in grammar can lead to lifelong difficulties, making it essential to determine the extent to which impairments may be pervasive among different groups of children. Although impairments in grammar have primarily been associated with DLD, there has been a growing body of research that has demonstrated that difficulties with grammar, and tense marking in particular, may be present across a variety of neurodevelopmental disorders, including FXS (Estigarribia et al., 2011; Sterling, 2018; Sterling et al., 2012). However, research comparing children with FXS to children with DLD has been limited and exclusively used norm-referenced assessments of tense marking accuracy (i.e., the TEGI; Haebig et al., 2016). Thus, the purpose of the current study was to extend this work by comparing and characterizing the tense and agreement productivity of boys with FXS and younger, MLU-matched children with DLD and TD using language samples. Moreover, this study compared children's pattern of tense marking and examined whether tense productivity was related to other measures of expressive vocabulary and grammar.

Comparison of Tense and Agreement Productivity in FXS, DLD, and TD

When examining the total TAP score, our findings revealed that children with DLD produced fewer overall tense and agreement markers than children with TD but that they did not differ from boys with FXS. Moreover, boys with FXS did not differ in their overall production of tense and agreement morphemes compared to children with TD. In other words, the overall tense marker production of boys with FXS fell in an intermediate range between children with DLD and TD, where the groups on the most extreme ends of the continuum (DLD at the low end and TD at the high end) differed from one another but not from the boys with FXS. Nevertheless, analyses of the individual tense markers revealed a slightly different and more complicated pattern of tense marking across these three groups. More specifically, boys with FXS and children with DLD demonstrated similar levels of productivity for all tense markers except auxiliary DO—children with DLD used auxiliary DO in fewer unique syntactic contexts than boys with FXS. Both boys with FXS and children with DLD produced fewer instances of third-person singular than younger, MLU-matched children with TD. Children with DLD also produced fewer distinct instances of auxiliary BE than children with TD. Interestingly, no other differences were found between any of the groups. Therefore, children with TD used auxiliary DO as productively as boys with FXS and children with DLD. Specifically, the children with TD produced auxiliary DO at an intermediate level between boys with FXS, who used it the most, and children with DLD, who used it the least. Additionally, children across the three groups did not differ in their use of copula BE or past tense –ed.

In some ways, this pattern of results aligns more closely with our alternative hypothesis and contrasts from the findings of Haebig et al. (2016), specifically in terms of the differences they found between children with DLD and FXS when using the TEGI to measure tense marking accuracy. In particular, Haebig et al. (2016) found that boys with FXS were more accurate than younger, MLU-matched children with DLD across all probes on the TEGI, not just DO probes. Moreover, their analyses revealed that boys with FXS were in line with younger, MLU-matched children with TD on past tense, BE, and DO probes, and actually outperformed children with TD on third-person singular probes. This latter difference directly contradicts the direction of our findings. Given these discrepancies, it is important to consider that different assessment techniques were used (language sampling vs. norm-referenced assessment) and different dimensions of tense marking were examined (productivity vs. accuracy). Therefore, it is possible that boys with FXS may look more like children with TD than children with DLD in terms of their tense marking accuracy. However, a more nuanced pattern of tense marking may emerge when examining productivity using language sampling, where boys with FXS look similar to children with DLD for some tense markers and more like children with TD for others.

Differences in measurement may also explain the fact that children with DLD did not differ from children with TD across all tense markers, but just in terms of third-person singular and auxiliary BE productions. Typically, studies using more traditional measures of tense marking in obligatory contexts have found differences between preschool-age children with DLD and same-age and/or MLU-matched peers in their use of all tense and agreement marker types (e.g., Haebig et al., 2016; Leonard et al., 1992; Rice & Wexler, 1996). However, when examining tense using TAP scoring, Gladfelter and Leonard (2013) also found that 4- to 5-year-old children with DLD had a lower overall TAP total score, but did not differ from age-matched children with TD in their productivity levels for copula BE and past tense –ed. Thus, the results of Gladfelter and Leonard map onto the findings in our study. As such, researchers and clinicians should be cognizant of the fact that the type of measurement selected may influence the profile of tense marking they obtain, underscoring the limitations of relying on one measurement type. Nevertheless, we believe these results complement prior research using other assessment techniques and help to provide a more complete profile of tense marking in children with FXS and how it compares to DLD.

Examining Differences Across Individual Tense and Agreement Markers

The second aim of this study was to examine children's productivity patterns across the five tense and agreement types. All children, regardless of diagnostic group, generally used copula BE more productively than all other tense markers, with the exception of third-person singular, which they produced at a similar rate. This finding is not surprising given that copula BE and third-person singular are known to be the earliest acquired tense morphemes (Brown, 1973) and others have found them to generally be the most productive forms when using TAP scoring (Gladfelter & Leonard, 2013; Hadley & Short, 2005; Rispoli et al., 2012). Also similar to previous research (Gladfelter & Leonard, 2013; Hadley & Short, 2005), children with DLD and TD used third-person singular more productively than past tense –ed, auxiliary DO, and auxiliary BE. In contrast, for boys with FXS, there was not a significant difference in their productivity for third-person singular, past tense –ed, auxiliary DO, or auxiliary BE. Instead, they demonstrated a more even profile across these four tense markers. There are a couple of possible explanations for this distinctive profile in boys with FXS. First, it may be that boys with FXS show phenotypic differences in the developmental sequence of individual tense markers compared to children with DLD or TD. Second, while there was not a significant relationship between TAP performance and chronological age, it may also be that the boys with FXS had more experience using some tense markers (e.g., auxiliary DO) as a result of being chronologically older. Finally, it is possible that differences in sampling context (play-based vs. semistructured conversation) could have influenced tense marker production, though sampling procedures were considered to be age appropriate for the respective groups and both styles were conversational in nature.

Relations Between TAP Performance, Age, IQ, and Other Expressive Language Measures

Consistent with previous research (e.g., Rice et al., 2004; J. A. Roberts et al., 2004; Sterling et al., 2012), we found that tense and agreement productivity was not tightly linked with nonverbal cognition. Similarly, tense and agreement productivity was generally not related to chronological age, though there was one exception (i.e., age was related to copula BE production in DLD). The limited number of associations with chronological age was somewhat unexpected given that some research has shown that both children with TD and DLD show growth in tense and agreement productivity during early childhood (Leonard et al., 2017; Rispoli et al., 2012), though this has not been found in all studies (Gladfelter & Leonard, 2013). Relatedly, others have demonstrated that boys with FXS show improvement on standardized assessments of grammar over time, albeit at a slower rate than boys with TD matched on nonverbal mental age (Martin et al., 2013). It is possible that there was not enough variability in chronological age among the children in our study, particularly those with DLD and TD, to show similar developmental differences in tense and agreement productivity. Nevertheless, these findings further support our decision to not include age and nonverbal ability as covariates.

We also examined the relationships between the TAP total score and other measures of expressive grammar and vocabulary. As expected, based on previous research (Hadley & Short, 2005), children in all three groups demonstrated a consistent profile of expressive grammar across the measures taken from language samples—that is, those who used tense and agreement markers more productively had longer MLUs. Although significant for all groups, the association between the TAP score and MLU was stronger in children with TD and boys with FXS (r ≥ .70) compared to children with DLD (r = .46). Thus, tense marking, while related to MLU, may not be as tightly connected in children with DLD. Similarly, lexical diversity measured via NDW was related to the TAP total score in boys with FXS and children with TD, but not in children with DLD. Thus, for children with FXS and TD, those with better expressive vocabulary abilities had better expressive grammar abilities. In contrast, lexical diversity was not related to tense productivity in children with DLD, which may suggest that the tense marking difficulties present in DLD are so significant that there is no relationship between NDW and productivity. Although several studies have found a correlation between grammar and vocabulary (Blom & Boerma, 2019; Hadley & Short, 2005), there is some prior work to suggest that finiteness marking and vocabulary development follow different trajectories in children with DLD and that performance on finiteness marking tasks is not predicted by vocabulary ability (Rice et al., 2009). Unlike MLU and NDW though, there was not a significant link between the TAP total score and the TEGI. This finding further underscores that these assessments are tapping into different dimensions of tense marking (productivity vs. accuracy) and that both types of measurement should be used to gain a comprehensive profile of tense marking abilities.

Limitations and Future Directions

Although findings from this study deepen our understanding of tense marking in DLD and FXS, several limitations must be discussed. First, the three groups differed significantly in terms of sex. Given that FXS is an X-linked disorder and girls with FXS can be, but are not always, less affected (Hagerman & Hagerman, 2002), only boys with FXS were included in this study. Although boys tend to have slightly higher rates of DLD (Whitehouse, 2010), sex differences are not as pronounced in DLD, and so studies commonly include both boys and girls (e.g., Gladfelter & Leonard, 2013; Guo & Eisenberg, 2014; Leonard et al., 2017). Nevertheless, future research comparing children with DLD and FXS could benefit from more closely matching on sex. Second, though the Leiter International Performance Scale–Revised and the RIAS are both correlated with other overlapping measures of IQ (e.g., both correlated with the Wechsler Intelligence Scale for Children; Reynolds & Kamphaus, 2003; Roid & Miller, 1997) and others have used different IQ measures to assess cognition within their samples of children with and without developmental disorders (Fortunato-Tavares et al., 2015; Haebig et al., 2016; Pickles et al., 2009), it is unclear whether there is concurrent validity between these two measures of nonverbal IQ. Third, we chose to measure TAP performance based on the first 100 utterances. It is possible that longer samples may have provided an opportunity for greater productivity and allowed for more pronounced distinctions between the groups to emerge. Moreover, though language sampling methods were selected for several reasons (e.g., age appropriateness), language samples differed between the boys with FXS and children with DLD and TD, which could have influenced language performance. Finally, in order to more fully understand the grammar profiles of DLD and FXS, it will be important for research to compare these clinical groups utilizing other assessment techniques (e.g., frequency of tense errors in natural language samples) and to examine whether different trajectories of tense marking emerge over time.

Conclusions and Clinical Implications

Although additional work is needed to understand the unique language phenotypes of FXS and DLD, the findings from this study further elucidate the areas of commonality as well as distinction in tense and agreement productivity among boys with FXS and DLD. Specifically, we found that children with FXS and DLD used some tense markers (e.g., third-person singular) less productively than MLU-matched children with TD and that boys with FXS were generally similar in tense and agreement productivity compared to children with DLD (with the exception of auxiliary DO). However, boys with FXS demonstrated a relatively even profile of productivity across the tense and agreement types, whereas children with DLD and TD showed a pattern of most-to-least productive tense morphemes that generally looked like that reported by others (Gladfelter & Leonard, 2013; Rispoli et al., 2012). In terms of clinical implications, our findings highlight the importance of using multiple techniques to determine the degree to which tense marking profiles differ in boys with FXS compared to children with DLD and children with TD. Such procedures will also be essential for determining the degree to which language intervention focusing on finiteness may need to be uniquely tailored to each clinical group.

Acknowledgments

We would like to acknowledge the funding sources that supported this project: R03 DC011616 (awarded to Sterling), U54 HD090256 (awarded to Chang), T32 HD07489 (awarded to Hartley), F31 DC009135 (awarded to Hoover), and T32 DC000052 (awarded to Rice), as well as start-up funds from the University of Wisconsin–Madison (awarded to Sterling). We would like to thank the children and families who participated in this research, as well as the lab members who contributed to this work, with particular thanks to past and present members of the Research in Neurodevelopmental Disabilities Lab at the University of Wisconsin–Madison and the Sounds 2 Syntax lab at the University of Massachusetts Amherst.

Appendix

Examples of TAP Scoring for Each Tense Type

Examples
Copula BE
 All instances of copula BE in the sample: Scoring (out of 5) a
  C: Is this a frog? 1
  C: Those are silly ducks. 1
  C: The cows are not ready. 1
  C: The cows are over there. 0 (same sentence subject)
  C: Is that some hay? 1
  C: All of the ducks are in the pond. 1
  C: The dog is brown. 0 (reached max score of 5)
 Total: 5
Third-person singular
 All instances of third-person singular in the sample: Scoring (out of 5) b
  C: He fits. 1
  C: He wants to ride. 1
  C: She wants to go. 0 (same verb)
  C: He just stands up on it. 1
  C: Abby calls him meow meow. 1
  C: It moves this. 1
 Total: 5
Past tense –ed
 All instances of past tense –ed in the sample: Scoring (out of 5) b
  C: I dropped it. 1
  C: He jumped up. 1
  C: I dropped her. 0 (same verb)
  C: I closed it. 1
 Total: 3
Auxiliary BE
 All instances of auxiliary BE in the sample: Scoring (out of 5) a
  C: I was riding in the back. 1
  C: Are you gonna put your feet up? 1
  C: She was spitting on me all day. 1
 Total: 3
Auxiliary DO
 All instances of auxiliary DO in the sample: Scoring (out of 5) a
  C: Do they go in here? 1
  C: Do you hear that? 1
 Total: 2
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Note. TAP = tense and agreement productivity.

a

Utterances with copula BE, auxiliary BE, and auxiliary DO were included in the TAP score if they occurred with different sentence subjects.

b

Utterances with third-person singular and regular past tense –ed were included in the TAP score if they occurred with different verbs.

Funding Statement

We would like to acknowledge the funding sources that supported this project: R03 DC011616 (awarded to Sterling), U54 HD090256 (awarded to Chang), T32 HD07489 (awarded to Hartley), F31 DC009135 (awarded to Hoover), and T32 DC000052 (awarded to Rice), as well as start-up funds from the University of Wisconsin–Madison (awarded to Sterling).

Footnote

1

DLD is also referred to as specific language impairment (SLI). However, SLI has stricter exclusionary criteria (i.e., a nonverbal IQ standard score of 85 or above) compared to DLD (i.e., a nonverbal IQ standard score of 70 or above). Many, but not all, studies in the literature that are now referring to children as having DLD would have previously described children as having SLI. The children included in this study meet criteria for both DLD and SLI and have been referred to as having SLI in previous studies by the authors. The classification of DLD was used in order to align with the current literature (see Bishop et al., 2017).

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