Second to fourth digit ratio: ethnic differences and family size in English, Indian and South African populations

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Abstract
Background: The second to fourth digit ratio (2D:4D) is lower in men than in women. 2D:4D may be established in utero and is negatively correlated with sperm counts and testosterone in men and positively correlated with oestrogen in men and women. Negative associations between male 2D:4D and family size and positive associations between female 2D:4D and family size have been found in some European populations. Aim: The study seeks to investigate the relationship between 2D:4D and family size in samples from Europe, Asia and Africa. Subjects and methods: Measurements of right hand 2D:4D were made in English (Merseyside, n = 403), Indian (Sugali and Yanadi, n = 160) and South African (Zulu, n = 258) populations. Results: 2D:4D was weakly sexually dimorphic and showed strong ethnic differences. There were significant associations between 2D:4D and family size. For males these included negative associations in English and South African samples, and in the total sample. In females there was a positive association in the English sample, a negative association in the South African sample, and the total female sample showed a positive relationship between 2D:4D and family size. Conclusions: 2D:4D showed weak sex and strong ethnic differences. Low values of 2D:4D were associated with large family size in men. The situation was less clear in women but on balance there was evidence to support a relationship between high values of 2D:4D and large family size.