Table 1.

Demographic characteristics of the twin sample^a

(mean ± SD)	Females (n = 199 individuals)	Males (n = 120 individuals)	Total (n = 319 individuals)
Age (years)	23.5 ± 1.8	23.7 ± 1.9	23.6 ± 1.8
FIQb	113.4 ± 10.9	117.5 ± 12.9**	114.9 ± 11.9
Gestational age (weeks)c	37.9 ± 2.8	38.1 ± 2.7	38.0 ± 2.7
Birth weight (g)	2575.4 ± 477.4	2715.9 ± 525.4*	2628.5 ± 500.0
Socioeconomic index	52.6 ± 24.2	54.7 ± 24.6	53.4 ± 24.3

^aThe sample included 75 MZ pairs, 66 DZ pairs, and 37 unpaired twins. Although unpaired MZ and DZ twins did not contribute to the estimation of the genetic and environmental parameters, they did contribute to the estimation of mean and variance effects (i.e. they allowed a more accurate estimation of phenotypic correlations and phenotypic effects).

^bFIQ was measured using five subtests from the Multidimensional Aptitude Battery (Jackson, 1984), as close as possible to the twins' 16th birthday. The observed higher mean is likely due to the fact that the MAB test was created and normalized for Canadian samples, and therefore results on this test may differ when used in a different country. In addition, the presence of an ascertainment bias cannot be excluded, as more intelligent and often more highly educated people tend to volunteer for these studies more frequently. However, the higher FIQ mean does not affect the representativeness of this sample because FIQ follows a normal distribution, with scores ranging from 85 to 146, thus showing good variability.

^cGestational age, birth weight, and parental socioeconomic status (McMillan et al., 2009) were obtained from parental reports, either when the twins were 12 or 16 years. There were no significant mean or variance differences between co-twins or by zygosity. Males had slightly higher FIQ (d = 0.34, p < 0.01) and birth weight (d = 0.28, p < 0.05) than females.

*p < 0.05;

**p < 0.01.