Women are attracted to more masculine-looking men at the most fertile time of their menstrual cycle, psychologists have shown.

During the less fertile times, they choose men with more feminine-looking faces. These are seen as kinder and more co-operative, but less strong and healthy genetically.

A controversial implication of the new research is that, in evolutionary terms, it is natural for a woman to be unfaithful in order to secure both the best genes and the best carer for her children.

This is because a less masculine-looking man may be a better long-term partner, but the strongest, healthiest children would be produced by a quick fling with a more masculine-looking man.

Not a moral judgement

However, the head of the laboratory at St Andrews University where the research was done, Professor David Perrett, told BBC News Online: "This suggestion is a possibility, but we don't know how behaviour is affected by the preferences we see. We're assuming that preferences for different faces are affecting the choices women make."

"But whatever is best in an evolutionary sense is not necessarily the moral thing to do socially. We are not advocating any particular strategy," he said.

The study was carried out by researchers in Scotland and Japan. They asked women to select the one face from a range that they were most attracted to as a partner for a short-term sexual relationship.

They found that in the most fertile week of their menstrual cycle, women preferred more masculine faces. However, the choice of face did not vary for women using an oral contraceptive (i.e. not fertile) or those asked to choose the most attractive face for a long-term relationship.

Smell of success

The results are supported by previous research which showed that a male hormone smells unpleasant to women, except in the week of fertility. Also, the smells of more symmetrical, and therefore more attractive, men are preferred by women but again only in that week.

Men who look more masculine have higher levels of male hormones and also show a better ability to fight off disease. This makes them attractive as potential mates because their children will inherit this useful characteristic.

Professor Perrett believes that preferences for certain types of faces will have an effect on the partners people choose: "We keep finding very strong links between the appearance of males and their perceived personality. People reckon they can judge personality from the way others look."

"And as long as those links are there, I think preferences will be a profound influence on choice," he said.

He also points out that there are real links between face form and behaviour. For example, a study has shown that more masculine-looking US servicemen are more likely to get divorced and be violent towards their partners.

Not real life

However, Dr Paula Nicolson, a psychologist at the University of Sheffield, thinks this kind should not be seen as applicable to everyday relationships. She will deliver a lecture next month to the British Psychological Society's conference called "Evolutionary psychology is not the answer to everything."

"The research uses experimental methodology which accounts for extraneous variables and for social context. So they find the essence of human nature, which in this case is to do with mating behaviour," she told BBC News Online.

"But this methodology is also a weakness because this is not actually how people live - decisions about choice of partner are made on a whole range of issues. I think the effect of facial preferences is probably lost in today's social context.

"It is important to look at human biology in a basic sense but even most biologists would admit that biology is not that clear-cut."

The study is published in the journal Nature. The initial research was carried out through BBC Tomorrow's World Magazine, co-ordinated by Damian Carrington.