The BBC's Christine McGourty reports: "The research could have important applications in medicine"

It is the first time that one gene has proven sufficient to change complex social behaviours so dramatically, the US researchers believe.

Tom Insel and Larry Young, at Emory University, have also recently done related studies in non-human primates and now plan to focus on humans.

Progress for mental illness

Virtually every kind of human mental illness is characterised by abnormal social attachments, but very little is known about the role that genetics has on forming social partnerships.

Dr Larry Young comments on his research

The team think their research could lead to greater understanding of this area, for example in in learning how social isolation can result from brain dysfunction.

"In psychopathologic disorders, there is a severe deficit in social behaviour and there could be genetic factors involved," Dr Young said.

"But it takes a lot of time to move from animal research into humans," Young warned. "And in human cultures we may find that experience and values have a lot more to do with behaviour."

Partners for life

The experiments, published in Nature magazine, used a gene from the prairie vole. Males of this rodent are faithful partners and attentive fathers.

"After mating, the male prairie vole forms a strong social bond. He prefers to be with that mate to the exclusion of all others," said Dr Young. "That pair nests together. When she has her babies he spends as much time with those babies as she does. He also defends the nest and they stay together for the next litter and the next and the next."

By contrast, the male mice used normally abandon the female immediately after mating and have no role in raising their offspring.

But by transferring a gene from the voles to the mice, the male mice became much more sociable to their mates, although occasional "extra-marital" affairs did still occur.

Patterns of the brain

The gene determines the pattern of particular hormone receptors in the rodent's brain. The hormone here is called vasopressin and was already known to have an effect on male social behaviours such as aggression and communication.

The position of vasopressin receptors in the brain is the key to this social behaviour

Dr Insel said: "What is really intriguing is that a change in a single gene can lead to a new pattern of receptors in the brain and then result in this profound difference in something as complex as social behavior."

Although a multitude of genes are likely to be involved in the evolution of monogamy, this work begins to identify the links between DNA sequences, brain chemistry and social behavior.

"Perhaps it will turn out that mutations in this same gene have occurred many times in evolution, leading to alterations in patterns of social interaction and facilitating monogamy under special socio-ecological conditions," Dr Insel added.