Throughout history, women have generally preferred men with masculine facial characteristics like broader faces, stronger eyebrows, chiseled cheekbones and strong jaw lines which are indicative of higher testosterone levels, but recently researchers proposed that women may be actually be most attracted to men with strong immune systems.

Scientists have known that there is a particular relationship between testosterone levels and immune functioning exhibited within many animals, however higher levels of testosterone have not always correlated with healthier immune systems.

Other researchers have suggested that there was a relationship between testosterone and healthy immune response may be influenced by the stress hormone, cortisol, which suppresses the immune system by “blocking” white blood cells from fighting off infections.

Lead author Dr. Fhionna Moore of Abertay University in the UK and her group tested both theory by measuring levels of testosterone, antibodies and cortisol of 74 Latvian men in their early 20s by taking their blood samples before and a month after participants were given their first dose of Hepatitis B vaccine. Then 94 Latvian women who were also in their early 20s rated photographs of each man on a 10-point scale of attractiveness.

The vaccine is supposed to trigger the participants’ immune response to produce antibodies, the higher the levels of antibodies in the blood, the healthier the immune system which would indicate lower cortisol levels.

Researchers found that men with a lower levels of the stress hormone, cortisol, also had higher levels of testosterone and were also perceived to be more attractive by women who rated pictures of their face.

In contrast, men with lower levels of testosterone had higher concentrations of cortisol, and were rated as less attractive by females. Researchers suggested this finding could also be because men with higher concentrations of cortisol suppressed their immune system more.

"The more antibodies a man produces in response to a vaccine, the more attractive his face," said Moore. "We also found that men's testosterone was related to their immune system and their facial attractiveness: the higher the testosterone, the stronger the immune system and the more attractive the face. Interestingly, this was qualified by levels of stress hormones: the relationships with testosterone were strongest in males experiencing low levels of stress."

Researchers said their results suggest that while female face preferences directly correlate with the health of the male immune system, stress hormones may also play a somewhat significant role in the level of male attractiveness.

"The idea is that there are always individuals with good genes who can afford to have high testosterone - to be highly attractive - without paying the cost for this. We need to understand the genetics of this," commented Dario Maestripieri, a professor of comparative human development at the University of Chicago in the U.S, according to Cosmos online.

"The research makes perfect sense, [and] the question makes perfect sense, so we need similar studies done on different traits. They focused on faces, but you could look at other things. We also need to understand more about the molecular mechanisms through which cortisol and testosterone interact," he added.

The latest findings, published on Tuesday in the journal Nature Communications, may be challenged by other studies that have suggested that female facial preferences for men could be determined by cultural cues over biological cues.

In 2010, psychologists at the Face Research Laboratory at the University of Aberdeen in Scotland measured nearly 4,800 women from 30 different countries on facial preferences. Researchers took into account the women’s facial preferences, the country where they came from, and the country’s national health index.

Psychologists had found that they could predict the degree of masculinity preferred by a woman based on her country’s World Health Organization statistics for mortality rates, life expectancy and the impact of communicable disease.

A woman preferred more masculine features in a man if she came from a country where poor health is particularly a threat to survival. In contrast, women with who preferred less masculine faces tended to live in the healthiest countries.

The 2010 study was published in the journal Proceedings of the Royal Society: Biological Sciences.