Pregnancy 'more likely with a lover' Lovers 'more likely to get women pregnant': British Psychological Society's annual conference in Brighton

An unfaithful woman is more likely to become pregnant by her lover than by her husband or regular partner because of a change in the type of orgasms she has, a study suggests.

The pattern of orgasms experienced by female mammals during sex with a 'non-regular' partner favours sperm retention, scientists said yesterday. In addition, during periods of infidelity this change was 'hostile' to sperm from the regular male partner.

Speaking at the opening day of the British Psychological Society annual conference in Brighton, Dr Robin Baker, of the School of Biological Sciences at Manchester University, said that there was an evolutionary advantage in the phenomenon because it ensured a better genetic mix.

Dr Baker said that females always ejected some of their partner's sperm. On average about 35 per cent of sperm was ejected within 30 minutes of ejaculation. But the amount was dependent on the pattern of female orgasms. The female was 'endowed with considerable flexibility in their manipulation of inseminates,' he said.

Orgasms that climaxed at any time between one minute before the male ejaculated and up to 45 minutes after, led to a high level of sperm retention. Lack of orgasm or an orgasm more than one minute before the male ejaculated resulted in a low level of sperm retention.

The study also concluded that male masturbation was a strategy to increase sperm fitness. Although it reduced the number of sperm inseminated during next intercourse, it did not lead to a reduction in the number retained by the female.

Dr Baker and a colleague, Dr Mark Bellis, analysed 'ejaculation data' for 35 human couples and evidence relating to 'the most recent copulation' reported by 3,587 women in a national survey.

The controversial theory on pregnancies in unfaithful females is largely drawn from studies involving sparrows. More pregnancies occurred in female sparrows impregnated by a non-regular partner than by a regular partner.

Professor Robin Dunbar of University College London, told the conference that males seeking long- term partners through the columns of small advertisements should promote their abilities as a parent and their sound family values. An analysis of more than 800 advertisements placed in two newspapers in the US - one broadsheet and one downmarket tabloid - revealed a shift in what women wanted.

Traditionally, women have sought financial security, someone who is able to look after them and their offspring. 'Resources are no longer as important,' Professor Dunbar said. 'Women are increasingly financially independent and can look after themselves. There is a shift to asking for good parenting abilities. Men haven't caught on to this yet.'

Men, in a ratio of two to one, still emphasise in the advertisements what they have to offer materially. Women in the same proportion are looking for a good family life.