In my edited book TWIN AND TRIPLET PSYCHOLOGY I discuss the factors that may contribute to differences in identical twins which I will summarise here:-

Our genes account for approximately half of the development of most psychological traits with non shared environment contributing to the rest. IQ has been shown to be much more gene dependent and Swedish studies have shown that this increases with age with 80-year-olds being much more alike in this respect. We need to be aware of the importance of the non-shared and shared environment as multiples go through life, the former creating differences and the latter making non-identical twins either more alike or sometimes less alike.

There are physical differences in utero that may favour one or other twin and that environment may be changed by the behaviour of the fetus. Inequality in the intra-uterine environment has an effect on each twin or triplet, leading to differences. Maternal bonding may be influenced by birthweight or birth order and preferential treatment, leading to differences in personality. Genes, though similar, may be activated at different times leading to physiological differences. THE COUPLE EFFECT -see previous blog, will also create differences between the pair.

This discussion has been prompted by Joan Friedman’s comments on twins proving non-identical for developing cancer. #twins #multiples #parentsofmultiples #fathersofmultiples #twinmoms #Adult twins