Women wanting to get pregnant should find themselves an ugly man, new research suggests.
Scientists have found attractive males produce less sperm during sex.
Researchers think good-looking males are biologically geared to hold back their sperm in each encounter to increase their chance of impregnating more females.
But unattractive males know they are not going to bed so many females — so when they do get lucky they give it all they've got.
The findings from the University of Oxford and University College London are backed up by studies of chickens and fish, but researchers think they could well apply to humans too.
"Human attractiveness is complicated and influenced by a number of factors including cultural preferences," said UCL researcher Sam Tazzyman. "Nonetheless, ejaculate size and sperm quality are likely to have been molded by similar forces, like attractiveness and the number of sexual partners, that are important in other species."
Reporting the findings on the university's Web site, Tazzyman said, "The more attractive a male is, the more females will be willing to mate with him, reducing the value of each mating to him. This means it is optimal for him to contribute fewer sperm per mating."