A unique long-term study of "test-tube babies" has confirmed that parenting styles as well as genes have major effects on how children turn out.
The study has found that angry and aggressive parents are more likely to have angry and aggressive children - even when the children have been conceived with donor eggs and sperm so they are biologically unrelated to their parents.
The children are also more likely to be aggressive if their biologically unrelated mothers are cold and distant, and they are more likely to be depressed if their unrelated mothers are depressed.
But if unrelated fathers are cold and distant or depressed, they have no perceptible effects on the children.
Otago University psychologist Gordon Harold, who led the study, said the findings would help target parenting programmes at parents' aggressive behaviour towards children such as shouting and getting angry.
"Interventions that target hostile behaviours from parents are likely to pay dividends in reducing behavioural problems in children," he said.
"If the parenting programmes don't work or we are not seeing sustainable effects, one argument could be that you are targeting the environmental aspects but it [the cause of children's behaviour] could be predominantly genetic. This study shows that it can't be solely genetic."
He said the age-old argument about whether "nature" or "nurture" had a bigger effect on children had been tested by many studies using twins or adopted children, but this was the first study of the issue using children conceived by in-vitro fertilisation (IVF) - literally "in glass" fertilisation, in test tubes.
Irish-born Professor Harold, who moved to Dunedin in 2008 and is returning to Britain this month for family reasons, was among a group of British psychologists who collected a sample of 888 IVF children born in Britain and the United States between 1994 and 2002.
Unlike adopted children, the IVF children could be separated into five groups. Some were biologically conceived by both parents, some by their mothers only with donor sperm, some by their fathers only with donor eggs, some as completely unrelated donor embryos, and some using surrogate mothers.
This meant the study was able to distinguish environmental and genetic effects for mothers and fathers separately for the first time.
Both parents were asked to rate themselves, their partners and their children on scales for depression, anti-social behaviour, parental warmth and parental hostility towards the children.
Professor Harold said the results showing correlations between unrelated parents and their children did not show that children were solely the products of their environments.
"What this paper is not saying is that genetic factors do not matter in human development. Absolutely not!" he said.
The latest results, published in the journal Psychological Medicine, were based on ratings when the children were aged between 4 and 10.