Genetics of Facial Masculinity 5 between obiective and rated masculinity (male faces:r= the correlations between nonidentical twin pairs (ie. .23.p<001;female faces:r=.25,p<001).Objective male-male,female-female,and male-female)did not differ sed only on shape an nd was not associ formnthecohtio2bcoecnorpo0die图 of 29)and acne (female faces:r=.29.p<001;male faces: dnm r=21.)and were presumably influenced by in 3.Int =0.04,P= 85,or ents measure correlated much more strongly with the compo- shown in Table 1.Correlations between identical twins were markedly greater than correlations between same ed shape variables (male faces: sex nonidentical twins and siblings for both male ces:r 1.92 sis that th and em Z(D =4.93.p=.05 available online for details of the analysis.For more nent of facial masculinity in both sexes.The estimated proportions of variation in facial masculinity due to genetic and reported and lem Statistical anabsis genetic factors.whereas virtually no variation was attrib Identical twins share all their genes,whereas nonidenti- uted to shared environmental influences.This finding is cal twins of their comple y environ for good genes environmental (C).and residual (sources.As is stan- One of our main goals was to determine the degree to liked for twin-family designsi we conducted maximum- The factt and twin or sibling pair c and sibl :sce Table D suggests that For further details on the type of twin analysis that we heritable factors that increase male facial masculinity also (2003) cond 2 ces bet means and correlations of different zygosity groups were Zygosity group r195%CIl thehangen mde罗 and test as Pa山 which All identical twins 50L305g Nonidentical female twins (n=113 pairs) 30[11,45 Results 16-.04.35 mean facial masculinit Male siblings (n=39 pairs) -09上382 of th ,221=248=20 230.3 Means of female or male members of same-sex pairs did iblings 231.09.36 not differ significantly from means of female or male mem- pairs.() 85,which Opposite-sex siblings (n=120 pairs) of nins Opposite-sex twins and siblings it Me did not differ significantly from means of other siblings ()=3.60,p=.17,which suggests that there was nothing unusual about the facial masculinity of twins.FurthermoreGenetics of Facial Masculinity 5 between objective and rated masculinity (male faces: r = .23, p < .001; female faces: r = .25, p < .001). Objective masculinity was based only on shape and was not associ￾ated with ratings of grooming or acne, whereas masculin￾ity ratings were associated with ratings of grooming (female faces: r = −.44, p < .001; male faces: r = −.05, p = .29) and acne (female faces: r = .29, p < .001; male faces: r = .21, p < .001) and were presumably influenced by cues other than shape, such as skin color and tone, heavi￾ness of brow, and facial hair. Our objective masculinity measure correlated much more strongly with the compo￾nent of the masculinity ratings that is captured by the landmark-based shape variables (male faces: r = .53, p < .001; female faces: r = .57, p < .001) than with the raw masculinity measure. (See the Supplemental Material available online for details of the analysis.) For more detail on the rating process and genetic analyses of observer ratings, see Mitchem et al. (2013). Statistical analysis Identical twins share all their genes, whereas nonidenti￾cal twins share, on average, half of their segregating genes, and all twins completely share the family environ￾ment. Therefore, we were able to partition the variation in scores into three sources: additive genetic (A), shared environmental (C), and residual (E) sources. As is stan￾dard for twin-family designs, we conducted maximum￾likelihood modeling, which determines the combination of A, C, and E that best matches the observed data (i.e., means, variances, and twin or sibling pair correlations). For further details on the type of twin analysis that we used, see Neale & Cardon (1992) and Posthuma et al. (2003). All analyses were conducted in the Mx software package, Version 1.54a (Neale, Boker, Xie, & Maes, 2006). As is standard in twin modeling, differences between the means and correlations of different zygosity groups were tested by equating the relevant parameters in the model and testing the change in model fit (distributed as χ2 ) against the change in degrees of freedom (which equals the change in the number of parameters estimated). Results Preliminary testing found that mean facial masculinity scores did not significantly differ between identical and nonidentical twins of the same sex, χ2 (2) = 2.48, p = .29. Means of female or male members of same-sex pairs did not differ significantly from means of female or male mem￾bers of opposite-sex pairs, χ2 (2) = 0.31, p = .85, which suggests that prenatal hormone transfer from one twin to the other had no influence on this trait. Means of twins did not differ significantly from means of other siblings, χ2 (2) = 3.60, p = .17, which suggests that there was nothing unusual about the facial masculinity of twins. Furthermore, the correlations between nonidentical twin pairs (i.e., male-male, female-female, and male-female) did not differ significantly from the correlations between corresponding nontwin sibling pairs, χ2 (3) = 2.18, p = .54, as expected given the equivalent genetic and environmental similarity of nonidentical-twin and sibling pairs; these correlations were equated in subsequent modeling. There was no significant effect of age on facial mascu￾linity scores in males, χ2 (1) = 0.04, p = .85, or females, χ2 (1) = 0.63, p = .43. Intraclass correlation coefficients are shown in Table 1. Correlations between identical twins were markedly greater than correlations between same￾sex nonidentical twins and siblings for both males, χ2 (1) = 11.92, p < .001, and females, χ2 (1) = 4.93, p = .03, which suggests that there is an important genetic compo￾nent of facial masculinity in both sexes. The estimated proportions of variation in facial masculinity due to genetic and environmental sources are reported in Table 2. For both males and females, approximately half of the variation in facial masculinity was attributed to additive genetic factors, whereas virtually no variation was attrib￾uted to shared environmental influences. This finding is consistent with the assumption that variation in facial masculinity is substantially heritable, which is a neces￾sary condition for facial masculinity to serve as a signal for good genes. One of our main goals was to determine the degree to which genes that affect masculinity in men have the same effect in women. The fact that facial masculinity scores were significantly correlated between opposite-sex twins and siblings (r = .23, p < .001; see Table 1) suggests that heritable factors that increase male facial masculinity also Table 1. Intraclass Correlation Coefficients for Objective Facial Masculinity of Twin and Sibling Pairs Zygosity group r [95% CI] Identical female twins (n = 110 pairs) .50 [.36, .61] Identical male twins (n = 88 pairs) .50 [.34, .62] All identical twins .50 [.39, .59] Nonidentical female twins (n = 113 pairs) .30 [.11, .45] Female siblings (n = 55 pairs) .20 [–.16, .46] All nonidentical female twins and siblings .28 [.11, .42] Nonidentical male twins (n = 93 pairs) .16 [–.04, .35] Male siblings (n = 39 pairs) –.09 [–.38, .22] All nonidentical male twins and siblings .09 [–.08, .26] All nonidentical same-sex twins and siblings .23 [.10, .35] Nonidentical opposite-sex twins (n = 171 pairs) .23 [.09, .36] Opposite-sex siblings (n = 120 pairs) .23 [.04, .39] Opposite-sex twins and siblings .23 [.12, .33] Note: Means and variances were equated across zygosity (within sex). Sibling pairs are not independent (i.e., a nontwin sibling can have a sibling relationship with each member of a twin pair). CI = confidence interval. Downloaded from pss.sagepub.com by Cai Xing on January 7, 2014