Scientists say the idea of mothers and fathers forming faithful partnerships may not be how nature intended us to evolve. In some societies at least, being unfaithful is actually good for your kids.

Delegates at the American Association for the Advancement of Science (AAAS) expo in Anaheim have been told about South American tribes where children can have as many as five "fathers".

"Among these tribes, it's believed that any man who has sex with a woman around the time she gets pregnant, or at any time during the pregnancy, contributes to the formation and the development of the foetus, and thus all those men share the biological paternity of the child," says Stephen Beckerman from Pennsylvania State University.

Survival rate

Beckerman's research has shown that the children that have more than one "father" actually have a better survival rate than those who do not.

This is probably because the secondary "fathers" either make additional contributions of fish and game to the children or somehow protect them from the dangers of childhood life.

The research work has identified 18 South American tribes in which there appears to be a licence for extramarital sex.

The Anglo Saxon view of things will find this hard to grasp. But if it benefits the children, there may well be evolutionary advantages to the arrangement.

Sperm wars

At the same symposium, writer Robin Baker, formerly of the University of Manchester, UK, presented his paper "Sperm Wars," looking at a more modern society's approach to the same issues.

He said his study showed modern women were just as likely to cheat on their long-term partners as their counterparts in the Amazon.

He said his research into the sexual habits of women in England found that 10% of children had a biological father who was not their mother's partner at the time of conception.

He claimed one in every 25 children were conceived in "sperm warfare," where conception took place while the mother contained the sperm of more than one man.

Baker claimed the majority of women would cheat on their long-term partners at least once in their reproductive lifetime, with infidelity more likely during ovulation.