Women find thin men more attractive as potential partners rather than those who look 'macho', according to a new study.
Macho features have long been touted as an evolutionary asset that heterosexual women look for in a potential mate but researchers said weight may be a more powerful driver of attraction as they found testosterone levels were more closely linked with weight than with macho looks.
Researchers in South Africa found that while women do respond more favourably to the faces and bodies of men with strong immune responses, they seem to cue into fatness and thinness, not macho features, when making their judgements.
Fatness, or adiposity, "is an obvious choice for a marker of immunity because of its strong association with health and immunity," study researcher Vinet Coetzee, a postdoctoral scientist at the University of Pretoria in South Africa, told LiveScience.
Macho features such as a strong jaw and squinty eyes advertise that a guy possesses high testosterone, according to the immunocompetence handicap hypothesis.
The trouble with the immunocompetence handicap hypothesis is that masculinity is not universally attractive to women, Coetzee and his colleagues wrote in the journal Proceedings of the Royal Society B.
Weight is consistently linked both to health and immune system functioning, Coetzee said.
To test the evolutionary role of fat, researchers photographed 69 Caucasian male volunteers in underwear. They also measured the men's body fat and testosterone levels.
About 65 per cent were healthy weight, 4 per cent were underweight and 30.4 per cent were overweight or obese.
The men's immune system response was also measured with a blood test done before and after they received a vaccine for hepatitis B.
Men with strong immune responses showed more antibody production after the vaccine than men with weak immune systems. Antibodies are the proteins that recognise and help neutralise foreign invaders in the body.
Next, 29 heterosexual Latvian women looked at photographs of the men's faces and bodies separately and judged them on attractiveness.
A separate group of 20 heterosexual Finnish men and women rated the men for masculinity, and 14 other Latvian women rated the men's facial fatness, or adiposity, which is highly related to overall body fatness.
The results showed that fatness, as measured with facial adiposity, was linked to both antibody response and attractiveness, with pudgier men both having weaker immune systems and being seen as less appealing by the fertile women.
A statistical analysis found that contrary to what the immunocompetence handicap would suggest, masculinity was not linked to either immune response or bodily or facial attractiveness.
"We found that a man's weight serves as a better indicator of the relationship between immune response and attractiveness than masculinity does," Coetzee said.