Washington, July 29 - An analysis of rare genetic disorders in which children lack some genes from one parent shows that maternal and paternal genes engage in a subtle tug-of-war well into childhood.

This striking new variety of intra-family conflict is the latest wrinkle in the two-decades-old theory known as genomic imprinting, which holds that each parent contributes genes that seek to nudge his or her children's development in a direction most favourable.

'Compared to other primates, human babies are weaned quite early, yet take a very long time to reach full nutritional independence and sexual maturity,' said study author David Haig, professor of organismic and evolutionary biology at Harvard University.

'Human mothers are also unusual among primates in that they often care for more than one child at a time. Evidence from disorders of genomic imprinting suggests that maternal and paternal genes may skirmish over the pace of human development,' he said.

'Clinical data from imprinting disorders suggest paternally-expressed genes promote, and maternally-expressed genes inhibit, childhood growth,' Haig writes.

'This analysis suggests that human life history, and especially humans' unusual extended childhood, may reflect a compromise between what's best for mothers, fathers, and the offspring themselves,' Haig said.