«Why are some people left-handed?». BBC Science Focus Magazine (på engelsk). Besøkt 30. oktober 2020. «Being left-handed is the result of genes and environment. About 50 per cent more males than females are left-handed and 17 per cent of twins are, compared with about 10 per cent in general. The ‘vanishing twin’ hypothesis suggests that left-handers were originally a twin, but the right-handed foetus failed to develop.»
