If you thought the mating business was already a jungle, where the pitfalls are looks, social rank, purchasing power, verbal skills and even subconscious smells, get ready to be dismayed for it is even more complex than thought.
Scientists in Germany have discovered that men who are stressed make an unconventional choice in sexual preference, says a report on Wednesday in the journal Proceedings of the Royal Society B, published by Britain's Academy of Sciences.
Previous studies have found piles of evidence that men and women who fall in love tend to resemble each other in many ways, especially in facial characteristics.
Acting on a hunch gained from experience with lab animals, psychobiologists led by Johanna Lass-Hennemann at the University of Trier explored whether a man responded differently to a woman when he was stressed.
They recruited 50 local students, all slim and healthy, and divided them into two groups.
One group was subjected to stress, in which they had to immerse their hand in icy water, while the other was unstressed, immersing their hand in water that was body temperature.
Saliva tests, monitoring for a hormone called cortisol, afterwards confirmed that the treatments resulted in different stress levels.
All the students were hooked up to electrodes measuring tiny muscle movements around the eye, known as the "startle reflex" that we give when we look at an object of interest.
They were then asked to look at 40 computer images of young women -- 30 of them erotic nudes of girls gazing at the observer, and 10 of them neutral -- while the "startle reflex" spies did their job.
What the scientists had done was to use a computer programme to morph the faces of 10 of the nudes so that these girls bore a subtle resemblance to that particular volunteer.
Men who had not been stressed showed, as was already known, a marked preference to women whose faces most resembled their own.
But men in the stressed group bypassed the close-resembling women and showed a preference for women who were dissimilar to themselves.
The study points out that in everyday life, a stressed man may not necessarily shoot for a dissimilar partner after this first, split-second encounter. But it may increase the probability.
Lass-Hennemann's team speculates that the reason for this counter-intuitive male behaviour is Darwinian -- the drive to maximise reproduction.
Past studies suggest that people tend to find self-resembling faces more trustworthy than dissimilar faces. This would explain why men who pursue a long-term relationship plump for women who look like them.
But there is also evidence that people who are stressed have more short-term relationships than unstressed counterparts. The more sexual partners you have, the more chances you have of extending your genetic lineage, which would explain why stressed men buck the trend.
Explore further: Male Rivalry Increases when Females at Most Fertile, Say Researchers