Classical psychological twin studies have yielded in part equivocal and sometimes contradictory results. Besides the complexity of the problem, the delicate character of the diagnosis, and the rudimentary tools which were used, several other factors are underlying this situation: (1) insufficient systematization of the set-up and the careless design of the investigations; (2) lack of follow-up studies; (3) uncertain diagnosis of zygosity; in MZ twins no attention to the age of the ovum at the time of cleavage; (4) lack of consideration for antenatal and perinatal influences; (5) no attention to the typical circumstances linked to the twin situation. All this makes it difficult to balance nature against nurture on the basis of comparison between MZ twins, DZ twins, and singletons. Interactions may also appear between the effect of twinning and other factors such as the socioeconomic circumstances. A new investigation was therefore started where, besides the twins, a group of matched control singletons was constituted. The follow-up study is now completed up to the age of 5 years in 13 MZ and 20 DZ twin pairs (+ controls, that is to say, 99 children).
The children were observed and subjected to psychological tests at the age of 6 months and of 1, 2, 3, 4, and 5 years.