Behavioural genetics of early childhood: fears, restlessness, motion sickness and enuresis.

Seventy-nine pairs of same-sex twins were examined at the age of three years at a municipal clinic and their mothers were interviewed to assess the twins' current and past behaviours. The zygosity was determined after the interview by fingerprints and/or bloodtyping in the majority of cases. The following significant differences in concordance between monozygotic and dizygotic twins were found: fear of strangers during observation at the clinic; marked fear of strangers in the first year of life; whether or not the child was startled by sudden noises during infancy; whether or not the child was able to sleep alone at three years without a parent sitting nearby; susceptibility to motion sickness; nocturnal enuresis; short attention span or restlessness during the test.