Genetic and environmental contributions to cranial capacity in black and white adolescents

Abstract

Data from 236 pairs of twins (472 individuals) aged 13 to 17 years were used to examine genetic and environmental factors influencing cranial size, an indirect estimate of brain volume. Measures were taken of zygosity, head lenght, head breadth, age, sex, race, height, and weight for 187 males and 285 females, 222 Whites and 250 Blacks. Cranial size was estimated from head length and head breadth using standard equations. Group differences were found. Cranial capacity increased over age 13 to 17 from 1,233 cm 3 to 1,279 cm 3. After adjusting for the effects of age and body size, boys averaged 1,290 cm 3 and girls 1,229 cm 3, Whites averaged 1,269 cm 3 and Blacks 1,251 cm 3. Intraclass correlations were calculated and models fitted of proportionate genetic and environmental contributions to variance. Depending on particular corrections for body size, heritabilities for the sample as a whole ranged from 38% to 51% with 6% to 20% due to common environment and from 42% to 52% due to unique (nonshared) environmental factors, including error variance. The proportionate contributions did not vary systematically by sex and the seemingly higher range of heritabilities estimated for Whites than for Blacks (47% to 56% against 12% to 31%) and the lower range of common environment effects for Whites than for Blacks (28% to 32% against 42% to 46%) did not differ significantly. In conclusion, it is indicated that genetic factors are required to account for the phenotypic variance in cranial capacity and that further research is required on whether environmental factors exert more influence in Black populations than in White populations.