What gives a guy the best chances of attracting a girl? Should he be metrosexual or macho?
Before reaching for the moisturiser or working on his abs, a man may be advised to head for an unusual source of guidance: the UN's databank of national health statistics.
In a study published on Wednesday, scientists report that women living in countries with worse rates of disease and ill health are far likelier to plump for "masculine"-looking men than "feminine"-looking rivals.
Psychologists from Scotland's University of Aberdeen tested a theory that masculine traits in men are a sign of genetic health.
If true, this should also have an effect on women, whose choice of a mate is influenced by the chances of their offspring's survival, according to the hypothesis.
The team recruited more than 4,500 women in their early twenties in 30 countries via the Internet.
They asked the volunteers to decide which of two faces of the same man was more attractive. Twenty pairs of photos had features morphed to look either square-jawed and masculine, or slim-jawed and feminine.
Preference for hunky types rose sharply as the country's "national health index" (NHI) fell, the investigators found.
The NHI is a basket of eight World Health Organisation (WHO) indices covering mortality rates, life expectancy and communicable diseases.
"Our prediction was correct," lead researcher Lisa DeBruine told AFP. "Women from countries with poorer health did prefer masculine men more."
The finding shows that mating choices may differ according to country and culture, but this is only because of variations in the local environment, said DeBruine. Underneath, women share a similar biological drive.
In previous research, DeBruine found that women who were more easily disgusted by cues of pathogens (such as men with open sores) were also likelier to prefer masculine-type fellows.
Butchness, though, is only part of a complex equation when it comes to mating.
A woman in an unhealthy, germ-laden environment may prefer a macho man as a sign of genetic wellbeing -- but she may not see him as a good bet for bringing up her children.
"Indeed, there is compelling evidence that women ascribe antisocial traits and behaviours to masculine men," says the study, published in the journal Proceedings of the Royal Society B.
"Women perceive masculine men as dishonest, uncooperative, more interested in short-term than long-term relationships, and even as 'bad parents'."
Other factors add to the complex mating mix.
During the most fertile, hormone-laden phase of her menstrual cycle, a woman is more likely to prefer masculine men. Social equality and control of resources can also shape perceptions about male attractiveness.
The study participants came from countries in which Caucasians are in the majority, ranging from low-income economies in eastern Europe such as Russia and Romania, to Canada and the United States.
Participants gave their ethnicity as White, as were the faces that were shown to them.
The researchers said they chose a single ethnicity in order to get a clearer signal from the respondents.
"Sexually dimorphic (distinct) facial cues have greater effects when women judge own-race faces than other-race faces," the paper said.
DeBruine said she hoped to extend the research to countries with very poor national health.
This was hard to do at present as these countries tend to have poor Internet access, which made it hard to get a representative cross-section of the population.
Explore further: Male Rivalry Increases when Females at Most Fertile, Say Researchers