While there is no "gene for caring", many species take more care of their immediate children because they have more of the same genes than strangers - including unrelated children. But that is causing problems in the modern age, said Stephen Emlen, professor of biology at Cornell University, New York, at the American Association for the Advancement of Science conference yesterday.
Professor Emlen said: "The nuclear family is becoming less common, replaced by growing numbers of single-parent and stepparent families, and at the same time we are seeing an increase in child abuse, delinquency and truancy."
But he said that by making people more aware of this predisposition - through education programmes similar to those about genetic disease - such problems could be lessened, or averted. "Be aware that if you are in a stepfamily, or dealing with one, there is a statistically greater chance of problems," he warned.
He also suggested involving grandparents in child-rearing,especially for single-parent families, and suggested that tax incentives might induce people to take part.
Studies of more than 300 bird species and 80 kinds of mammals, including humans, have shown that parents and grandparents tend to help their children, effectively protecting the genes that they have passed on to them.
But though this worked well for humans ever since the hunter-gatherers on the African plains 4 million years ago until the middle of the 20th century, that has recently begun to break down, said Professor Emlen.
"All else being equal, the closer the kinship, the greater the tendency for animals to co-operate," he noted. But he found in a survey of social scientists' reports that there was a correlation between the structure of the family and children's well-being.
He found that stepchildren are more often physically or sexually abused, or killed, than children in intact families. Stepparents spend less time and effort with their partner's previous children than with their own, and are far more likely to commit sexual abuse: the incidence of sexual abuse of stepdaughters in one study was eight times greater than for biological daughters.
Also, children in stepfamilies leave home earlier than children in intact families, while the marriage itself tended to be less stable too: divorce in American families was more common in second marriages, and that frequency grows with the number of stepchildren.
Professor Emlen commented: "The rules we evolved with don't work well in the greater diversity of family types present today."
"We didn't ask for these biological predispositions; they came as part of a bigger genetic package that worked just fine for our ancestors for thousands of years."Reuse content