Table 1 -.
Studies on physiological, morphological and reproductive traits in opposite-sex and same-sex twins
| Trait | Publication | Setting | Numbers of opposite-sex (OS) and same-sex (SS) twins | Is there a comparison between OS and SS dizygotic (DZ) twins only?a | Age | Assessment | Findings OS vs. SS females | Findings OS vs. SS males | Suggests evidence for masculiniza-tion of OS femalesb | Suggests evidence for demasculiniza-tion of OS malesb |
|---|---|---|---|---|---|---|---|---|---|---|
| Age at menarche | Kaprio et al., 1995 | Finland | 434 OSF 378 SSF (DZ) 468 SSF (MZ) |
Yes | 16 years | Self-reported age at menarche | OSF higher mean age at menarche than SSF (DZ) but not when compared with SSF (MZ) | NA | + | NA |
| Sorensen et al., 2013 | Denmark | 1147 OSF 1250 SSF (DZ) 1466 SSF (MZ) |
Yes | 12–22 years | Self-reported age at menarche | No significant differences | NA | − | NA | |
| Anthropome-tric measures | Gaist et al., 2000 | Denmark | 700 OSF 655 SSF (DZ) 703 SSF (MZ) 722 OSM 709 SSM (DZ) 734 SSM (MZ) |
Yes | Mean = 56.9 years | Self-reported weight and height, waist circumference and handgrip strength measured by interviewers | No significant differences | No significant differences | − | − |
| Anthropome-tric measures and metabolic aberrations | Alexanderson et al., 2011 | Sweden | 8,409 OSF 9,166 SSF |
DZ twins only | OSF: 42–93 years, mean = 59 years SSF: 42–103 years, mean = 61 years | Self-reported body mass index (BMI), hyperlipidemia and diabetes mellitus type II | BMI, body weight and rate of dyslipidemia moderately higher in OSF than in SSF but only in those over 60 years of age | NA | +/− | NA |
| Anthropome-tric measures, disease status and reproductive history | Korsoff et al., 2014 | Finland | Sample A: 789 OSF 679 SSF Sample B (subsample for serum lipid and lipoprotein subclass concentrations): 169 OSF 226 SSF |
DZ twins only | A: 32–37 years, mean = 34.0
years B: 21–29, mean = 24 years |
Obesity measures: BMI from self-reported
height and weight and waist circumference. Disease status: self-reported hypertension and diabetes. Serum lipid and lipoprotein subclass concentrations measured by nuclear magnetic resonance spectroscopy. Self-reported reproductive history (e.g. current pregnancy, number of children, abortions) |
No significant differences | NA | − | NA |
| Anthropome-tric measures | Bogl et al., 2017 | CODATwins 19 countries | 27,100 OSF 39,856 SSF 26,708 OSM 28,638 SSM |
DZ twins only | 20 years and older Males: median age = 44 years Females: median age = 42 years |
Height and weight mainly self-reported (97%), BMI calculated based on weight and height | No significant differences for BMI and overweight/obesity but OSF 0.31 cm taller than SSF | No significant differences for BMI and overweight/obesity but OSM 0.14 cm taller than SSM | − | − |
| Birth size, birth weight and gestational age | Tul et al., 2012 | Slovenia | 1097 OSF 1554 SSF 1097 OSM 1638 SSM |
No | Infants | Mean birth weight obtained from the Slovenian national information perinatal system | No significant differences | OSM heavier than SSM | − | − |
| Jelenkovic et al., 2018 | CODATwins 15 countries | 16,417 OSF 17,432 SSF 16,417 OSM 17,584 SSM |
DZ twins only | Infants | Mean birth weight, birth size and gestational age – mainly parentally reported | No significant differences | OSM on average 31 g heavier and 0.16 cm longer than SSM. OSM longer gestation than SSM | − | − | |
| Brain size | Peper et al., 2009 | The Netherlands | Sample A (children): 19 OSF 41 SSF 16 OSM 43 SSM Sample B (adults): 21 OSF 40 SSF 11 OSM 44 SSM |
DZ twins only | A: mean = 9.2 years B: mean = 30.0 years |
Magnetic resonance imaging (MRI) brain scans | Larger total brain and cerebellum volumes for OSF than SSF children. No significant differences in adults | Larger total brain and cerebellum volumes for SSM than OSM children. No significant differences in adults | +/− | +/− |
| Brain size and craniofacial morphology | Marečková et al., 2015 | Quebec, Canada | 35 OSF 28 SSF 36 OSM 20 SSM |
DZ twins only | 8 years | Magnetic resonance images (MRI) | Brain size: No significant
differences Principal components: 5/8 PC3-related distances smaller for SSF than OSF |
No significant differences | +/− | − |
| Circulating hormone levels/Cerebral lateralization (right ear advantage) | Cohen-Bendahan et al. 2004 | The Netherlands | 67 OSF 53 SSF |
DZ twins only | 10.4–11.8 years, mean = 11.0 years | Testosterone levels in saliva from radioimmunoassay. Auditory-verbal dichotic listening task | No significant differences regarding hormone levels. OSF greater right ear advantage of cerebral lateralization (more masculinized) than SSF | NA | +/− | NA |
| Circulating hormone levels/Handedness | Vuoksimaa et al., 2010a | Finland | Testosterone/estradiol levels 117/114 OSF 254/244 SSF 109/108 OSM 291/278 SSM Handedness 755 OSF 749 SSF (MZ) 706 SSF (DZ) 730 OSM 697 SSM (MZ) 784 SSM (DZ) |
No | 14 years | Testosterone and estradiol levels in saliva from radioimmunoassay. Self-reported handedness (based on two questions) | No significant differences regarding hormone levels. Lower prevalence of left-handedness in OSF than in SSF | No significant differences | +/− | − |
| Finger-length ratios (second to fourth finger; 2D:4D) | Van Anders et al., 2006 | Canada | 9 OSF 16 SSF 9 OSM 22 SSM |
DZ twins only | 4–15 years, mean = 10.2 years | 2D:4D measures from photocopies of hands | 2D:4D lower (more masculinized) in OSF than in SSF for the left-hand digit ratio but not the right | No significant differences | + | − |
| Voracek and Dressler, 2007 | Australia | 10 OSF 18 SSF |
DZ twins only | Adults | 2D:4D measures from photocopies of hands | 2D:4D lower (more masculinized) in OSF than in SSF | NA | + | NA | |
| Medland et al., 2008 | Australia | 212 OSF 237 SSF 199 OSM 219 SSM |
DZ twins only | 11–24 years, mean = 15.5 years | 2D:4D measures from photocopies of hands | No significant differences | No significant differences | − | − | |
| Hiraishi et al., 2012 | Japan | 9 OSF 150 SSF (MZ) 44 SSF (DZ) 9 OSM 58 SSM (MZ) 28 SSM (DZ) |
No | 16–32 years, mean = 22.5 years | 2D:4D measures from photocopies of hands | No significant differences | No significant differences | − | − | |
| Handedness | Elkadi et al., 1999 | Australia | 59 OSF 40 SSF 59 OSM 21 SSM |
DZ twins only | 18–26 years, OS: mean = 24.4
years SSF: mean = 24.7 years SSM: mean = 21.6 years |
Edinburgh Handedness Inventory questionnaire (preferred hand for 10 everyday activities) | No significant differences | No significant differences | − | − |
| Handedness/Footedness | Ooki, 2006 | Japan | Handedness Sample A: 125 OSF 138 SSF (DZ) 125 OSM 150 SSM (DZ) Sample B: 203 OSF 209 SSF (DZ) 203 OSM 182 SSM (DZ) Footedness Sample A: 113 OSF 130 SSF (DZ) 114 OSM 144 SSM (DZ) Sample B: 175 OSF 172 SSF (DZ) 177 OSM 150 SSM (DZ) |
Yes | A: 11–12 years B: 1–15 years, mean = 5.9 years |
Self-reported hand use for writing and foot use for kicking a ball (single question) | No significant differences in prevalence except for footedness for females in sample A | No significant differences | +/− | − |
| Handedness | Medland et al., 2009 | Australia and the Netherlands | 54,270 twins and their non-twin siblings (no breakdown provided) | Yes | Children and adults | Handedness assessed by writing/drawing or self-report | No significant differences | No significant differences | − | − |
| Leukocyte telomere length (LTL) | Benetos et al., 2014 | Denmark | 72 OSF 196 SSF (MZ) 176 SSF (DZ) 72 OSM 172 SSM (MZ) 132 SSM (DZ) |
Yes | Mean (baseline) = 36.0–39.8 years | LTL measured by Southern blots of the terminal restriction fragments | Shorter LTL in OSF than in SSF | No significant differences | + | − |
| Polycystic ovary syndrome (PCOS) | Kuijper et al., 2009 | The Netherlands | 480 OSF 711 SSF (DZ) 1325 SSF (MZ) |
Yes | Adults | Self-reported PCOS based | No significant differences | NA | − | NA |
| Reproductive function | Christensen et al., 1998 | Denmark | 4269 OSF 3650 SSF (MZ) 4455 SSF (DZ) |
No, but differences tested between zygosity groups | 18 years or older | Self-reported outcome of first try ever to become pregnant and waiting time to pregnancy | No significant differences | NA | − | NA |
| Reproductive function/Height | Loehlin and Martin, 1998 | Australia | Reproductive function: 600–700 OSF 1400–1500 SSF Height 685 OSF 1642 SSF |
No | Reproductive function: OSF: mean = 42.6 years SSF: mean = 39.7 years Height: OSF: mean = 39.7 years SSF: mean = 42.6 years |
Self-reported 90-item questionnaire related to reproductive functions and self-reported height | No significant differences | NA | − | NA |
| Lummaa et al., 2007 | Finland | 31 OSF 35 SSF 17 OSM 26 SSM |
No | Adults | Number of offspring obtained from Finnish Church registers | OSF had fewer offspring compared with SSF | No significant differences | + | − | |
| Medland et al., 2008b | Australia, the Netherlands and Virginia USA | 913 OSF 1979 SSF |
No | Adults | Self-reported number of children and age at first pregnancy | No significant differences | NA | − | NA | |
| Reproductive function, marriage, graduating from high school/completing college and socioeconomic outcomes | Bütikofer et al., 2019 | Norway | 13,717 twins including 6,808 female twins (breakdown not provided) | No | At least one twin observed to age 32 | Objective information on birth, household composition, schooling and labor market records from Norwegian Register Data | OSF decreased probability of graduating from high school (15.2%), completing college (3.9%) and being married (11.7%). OSF have lower fertility (5.8%) and lower life-cycle earnings (8.6%) than SSF | No significant differences | + | − |
| Tooth size | Dempsey et al., 1999 | Australia | 56 OSF 98 SSF (DZ) 166 SSF (MZ) 56 OSM 88 SSF (DZ) 132 SSF (MZ) |
No | 7–62 years, mean = 16.5 years | Tooth crown diameters of 28 permanent teeth recorded | OSF had larger tooth crown size than SSF for 26/28 teeth | No significant differences | + | − |
| Ribeiro et al., 2013 | Australia | 43 OSF 39 SSF (DZ) 52 SSF (MZ) |
Yes | 4–16 years | Tooth crown size determined by image analysis | Larger tooth crown size in OSF than in SSF | NA | + | NA |
NA: Not available, OS: opposite-sex, SS = same-sex, F = females, M = males, MZ = monozygotic, DZ = dizygotic, UZ = unknown zygosity
a
Yes: The study makes a separate comparison of OS vs SSDZ twins, although it includes both MZ and DZ twins.
No: The study does not make a separate comparison of OS vs SSDZ twins, thus, including MZ twins in all analyses.
DZ twins only: The study includes DZ twins only. Thus, no MZ twins are included in the study.
b
+: The study provides evidence for masculinization of OS females/demasculinization of OS males.
−: The study provides no evidence for masculinization of OS females/demasculinization of OS males.
+/−: The study provides evidence for masculinization of OS females/demasculinization of OS males in some cases (e.g. in some investigated measures, in some age groups or in some statistical analyses), but not in all.