The science of attraction
Sex may be an activity that takes place in the bedroom but its influence on our behaviour goes much further than the boudoir door.
While many people assume their choice to wear skinny jeans or grow a beard is based on personal taste or fashion trends, many scientists believe these decisions also reflect our primal need to attract a mate and breed to ensure the survival of our species.
Like a male peacock's brightly coloured tail feathers, humans also make grand displays of their talent and give off signals, often unconsciously, that attempt to lure a prospective partner. Lipstick, fast cars, Rolex watches and push-up bras may be arbitrary consumer goods, but they're also tools that can help humans catch a mate.
Just as Charles Darwin's theory of evolution by natural selection predicts that male mandrill monkeys evolved brightly-coloured genitalia to entice females, humans have also developed strategies to help us get lucky in the sack (or bathroom or cloakroom or back seat of the car or the boss' office, etc).
While scientists who study evolution have spent decades trying to understand the complex interplay between genes and culture and their influence on mate selection, research by a group at the University of NSW have discovered some interesting trends.
Ever since the ruggedly handsome Noah Calhoun pursued the girl of his dreams in the romantic film, 'The Notebook', women have swooned over actor Ryan Gosling.
The Canadian's features - a prominent brow, chiselled jaw and broad shoulders - epitomise masculinity. But why do women find these traits so darn hot?
"There's this idea that attractive faces are a good signal that the genes that control your immune system are good quality," says evolutionary biologist Rob Brooks, who runs the UNSW's sex lab. This unconscious message is thought to alert a prospective partner that the person would make a good mate because their robust genes would be passed onto the pair's offspring, he says.
But testing this idea in humans is tricky. In most animals, females play the role of selector. As the sex that invests the most effort in rearing their young, females need to be choosy when they pick a partner to sire their offspring. The male response is to compete.
While this interplay is more complex in humans - females can be rivals and men are picky too - the basic strategy remains.
In an attempt to identify broad patterns of human mate choice, specifically among women, Brooks and his team have scrutinised global data sets.
In developing countries such as Argentina and Mexico where healthcare is minimal and babies have a reduced life expectancy they found women preferred men with more masculine faces. In contrast, women in first-world nations such as Australia had a much weaker preference for these features, some even preferred men with feminine traits.
"Within every population there are women who like more masculine dudes and women who like more feminine dudes and all sorts of different preferences in between," Brooks says.
"But aggregated across a population you see these shifts."
In the regions where healthcare was basic, there were also higher rates of poverty, violence and inequality, which Brooks suspects is driving females' shifting mate preferences.
When men have poor prospects they are unlikely to be selected by women as mates, Brooks says.
Under these conditions there is a greater incentive for poor men to strive and out-compete other males for a female partner.
As males compete with other males, the stronger, more masculine ones father more children and their genes are passed on to future generations.
Cross the globe to the more benign, higher-equality conditions of eastern-suburbs Sydney, Australia, and the researchers found little difference between the prospects of the wealthy versus the poor. "The incentives to strive are not as strong," Brooks says.
And so without this need for males to increase their prospects, the selection pressure for masculine features decreases as women partner more feminine fellas.
"The rise of meterosexual men may be a reflection of the relatively equitable conditions of our society," he says.
Brooks says the drive of men with few prospects may be wrapped up in the development of other human traits, most notably music making.
He suggests rock'n'roll flourished, in part, because it was a way for young men with limited prospects to show off and elevate their status among women. How else does a bloke like Mick Jagger have seven kids with three international supermodels?
"Young men, in particular those with very poor prospects, have the scope to learn four chords and play their hearts out and elevate themselves from having almost no chance of scoring a mate to becoming a world-wide super star and reproductively prolific," Brooks says.
A similar situation may have played out in boxing rings.
"Muhammad Ali and just about every heavyweight boxing champion of the world has come out of working-class or poverty conditions and, largely on their own efforts, dragged themselves up to prominence," Brooks says. "And for every one man that has done that, there are hundreds who've failed or died trying."
While it can seem logical to view the widespread courtship rituals of humans as products of evolution, there is only limited experimental evidence to support these theories, says Paul Griffiths, a philosopher of science at the University of Sydney.
The problem with studying humans is the many variables in their environment.
"If you could have identical lines of humans bred in identical environments everything would be much easier, but you can't treat humans like lab rats," Griffiths says.
Brooks acknowledges the theories he and others have formulated around trends in mate selection are by no means definitive.
"It's an ongoing process," he says.
While there may have been a rise in skinny jeans-wearing, floppy fringed males such as Justin Bieber, you may also have noticed a rise in another male trend - facial hair.
Evolutionary biologist Barnaby Dixson has spent the past few years trying to understand the role of beards in mate selection.
In a recent study, Dixson, a post-doctoral research fellow at UNSW, showed heterosexual men and women pictures of clean-shaven, stubbled and full bearded men. He found women generally preferred stubble or clean-shaven men while heterosexual men rated beards attractive on other men.
Dixson suspects men subconsciously grow beards to declare their masculinity to other heterosexual men rather than an attempt to lure women. "It's probably more a male-to-male signal of age, and perhaps seen as a social dominance or even a threatening display [to other men] as opposed to an attractive signal to females," he says.
Previous research has shown that beards come in and out of fashion over time, and may be linked to changes in the environment.
"Masculinity becomes more important in an economy during a recession [that's when] beards tend to make a come back," Brooks says.
Dixson says there are plenty of anecdotal reports to suggest beards have made a comeback, but he knew of no research that supported their revival. Just because certain human behaviours evolved to assist selecting a mate, does not mean the trait cannot be changed or influenced by other factors, says Paul Griffiths, a philosopher of science at the University of Sydney.
Often the public assumes the evolution of a particular behaviour meant it was "determined by our genes and there is nothing we can do about it", which is not the case, Griffiths says.
It is an important distinction to make. Many social scientists and cultural anthropologists are hostile towards the field of evolutionary biology, believing it disregards the role thousands of years of human culture has played on our behaviours.
But as Brooks points out, learning and imitation of our peers shapes human behaviour also, changing biological connections in the brain.
"The aim of my research is partly to try and break down this ridiculous notion that things are either genetic and evolved or are environmental and inherently cultural," he says. "Everything is a mix of both."