It is every parent’s nightmare: your toddler bites another child and is too young to say sorry.
You may feel that your parenting is to blame or that the boisterous nursery environment is turning your child into a bully. However, scientists now suggest that genetic factors largely explain toddler aggression and that external factors may have little influence.
Eric Lacourse, a psychologist at the University of Montreal who led the study, said: “Parents should feel less guilty about this and be more modest about their power to influence their children.”
Physical aggression, such as biting, scratching and kicking, is a natural developmental phase, he said. “It’s normal to be aggressive at this age, but there are big differences, even within families.”
The study, published today in the journal Psychological Medicine, involved 667 pairs of twins, roughly half of whom were identical and half fraternal. Their mothers were asked to rate the twins’ aggression, by reporting behaviour such as hitting, biting and kicking, at 20, 32 and 50 months.
Comparing the incidence of aggression in identical twins with fraternal twins allows scientists to investigate the extent to which genetics influence this behaviour. Identical twins share 100 per cent of their genes, whereas fraternal twins share on average only half of the genes that differ between people.
The study suggested that at least 60 per cent, and probably more, of the variation between individuals was explained by genetic factors.
The authors said most toddlers would either learn or grow out of aggressive behaviour and there was no evidence that aggressive toddlers were more likely to become belligerent adults.
The study challenges popular parenting books, many of which suggest that toddler behaviour is largely governed by external factors.
However, Dr Lacourse said: “When a one-year-old bites, it’s almost like an automatic response in some cases.”