An eight-year study of Manchester University students, young and old, showed those with bigger gonads had sex more often, were more likely to be unfaithful to their long-term partner and had higher sperm counts.
Conversely, men with smaller testicles "spend much more time with their partners and make it difficult for them to be unfaithful", said Dr Robin Baker, a zoologist. They had lower sperm counts and engaged in sexual intercourse less frequently.
Dr Baker, who headed the study of 80 men aged between 18 and 50, says these findings suggest men are genetically disposed towards one of two sexual strategies thrown up by millions of years of evolution.
The well-endowed subconsciously incline towards promiscuity, while the little men lean - without ever recognising why - towards fidelity and staying close to their partners.
There are big differences in testicle size among the population. The largest in his survey, measured using callipers, had a volume of 52 cubic centimetres compared with the smallest at 8cc. The average was 24 cubic centimetres, considerably less than the 40cc average found in a Danish study.
His investigation took so long because it was not easy to find a large enough number of volunteers willing to be studied. Apart from the indignity of being measured, they also had to supply semen ejaculated in a condom during copulation. This was needed to measure their sperm count.
While his theory of two evolutionary strategies, big or small, swinger or faithful, may sound implausible, numerous studies on animals have demonstrated the evolution of all manner of different sexual strategies.
There is fierce competition between males to be the first to get their sperm to fertilise eggs.
Dr Baker estimates that 4 per cent of conceptions of human children involve such sperm races, in which the sperm of either of two men could have reached the fertile egg first.
The male seed remains viable inside the women's body for up to five days. It is broadly accepted that about 10 per cent of children are not related to the men who believe they are their fathers - suggesting that there is plenty of opportunity for such races.
Dr Baker also compared the sex lives of men with their degree of bilateral symmetry - how closely the right side of their body matched their left - and how attractive they were to women.
Again, the more symmetrical men had more sexual partners, more sex, and higher sperm counts during copulation.
There is a strong match between degree of symmetry and attractiveness to women, and a growing number of biologists believe that symmetry, and thus handsomeness, have an evolutionary meaning. Women, they theorise, subconsciously use it as a cue for the genetic fitness of prospective mates.Reuse content