Table 1.

Results of t-tests Examining Sex Differences in Mean Level of Behavioral Inhibition and Language Abilities.

	Girl n	Boy n	Girl Mean(SD)	Boy Mean(SD)	t-value
Observed Behavioral
Inhibition (range: 1–3)
14 month	390	400	1.94(.49)	1.83(.49)	−2.51*
20 month	348	365	1.98(.54)	1.87(.51)	−2.29*
24 month	352	362	1.96(.51)	1.91(.51)	−1.21
Parent-Reported
Behavioral Inhibition
(range: 0–1)
14 month	359	366	.49(.13)	.47(.13)	−2.29*
20 month	318	326	.51(.13)	.49(.14)	−1.48
24 month	335	341	.50(.13)	.49(.14)	−0.99
Expressive Language
14 month (range: 6–35)	386	391	21.11(3.86)	20.14(3.64)	−2.90**
20 month (range: 12–44)	340	358	31.20(5.86)	28.96(5.74)	−3.96**
24 month (range: 16–50)	339	344	39.08(6.76)	36.34(6.64)	−4.24**
Receptive Language
14 month (range: 3–38)	386	393	19.03(5.89)	15.97(5.69)	−5.28**
20 month (range: 12–46)	341	351	36.33(5.77)	33.67(6.94)	−4.09**
24 month (range: 22–54)	332	333	45.63(4.08)	43.72(4.98)	−4.37**

Note.

⁺p< 0.10,

^*p < 0.05,

^**p < 0.01.

Analyses corrected for nonindependence of twin data.

SD = standard deviation. Expressive and receptive language raw means are displayed for ease of interpretation; analyses were conducted on transformed scores due to non-normality of data.