Figure 1.

Distribution of siblings in the male and female sample with their relative zygosity (Not Twin, Dizygotic, Monozygotic). Groups’ abbreviations: m (males with no siblings); f (females with no siblings); m:m (males who have at least another male sibling); f:f (females with at least another female sibling); m:f (males with at least a female sibling); f:m (females with at least a male sibling); m:f+m (males with at least a male and a female sibling); f:m+f (females with at least a male and a female sibling).