When we talk about animal instinct, we're often talking about pheromones - chemicals that affect the behaviour of other animals, and make individuals irresistible to the opposite sex during mating season.
For decades, debate has raged over whether humans might also release and be susceptible to our own unique pheromones. But a new study has just provided evidence that the two leading 'human sex pheromone' candidates aren't actually pheromones at all.
Until now, researchers had narrowed down the hunt for human pheromones to two chemicals - androstadienone (AND), which is found in male sweat and semen, and estratetraenol (EST), found in women's urine.
The jury has remained out on whether or not they're true human pheromones, but that hasn't stopped the media and perfume makers from running with the idea, leading many people to believe to some extent that, as a species, we're responding to each other's subtle pheromone cues.
But in a new double-blind study, researchers from the University of Western Australia tested the effect on 94 healthy humans, and found that these so-called pheromones had no measurable impact on their behaviour whatsoever.
"Much of the research currently promoted focuses on studies that back AND and EST being pheromones in humans, because of the human fascination on how we can improve our attractiveness to the opposite sex," said lead researcher Leigh Simmons.
"This contributes to a skew in public perception on whether humans do have pheromones with many people believing we do, because research suggesting the opposite tends not to be as published, and if it is published it does not get the same degree of attention."
In the latest study, the team took 43 males and 51 females, and asked them to complete two computer-based tasks twice, on two separate days.
All participants were heterosexual and Caucasian, and had no idea they were being tested on anything related to pheromones. Neither did the examiners conducting the test, which makes this a double-blind study - the gold standard of research design.
During the first computer test, participants were asked to indicate the gender (male or female) of five gender-neutral facial images.
The second test asked participants to look at faces of the opposite sex and rate how attractive they found them, and how likely they thought they'd be to cheat on their partner.
They repeated these tests on two separate days. On the first day, they were exposed to a control scent through a cotton ball taped under their nose, and on the second day, they were exposed to either AND or EST.
The hypothesis was that if the chemicals they were exposed to on day two really were pheromones, then female volunteers who'd been exposed to the so-called sex pheromone AND would be more likely to rate gender-neutral faces as male, and rate the men in the second task as more attractive.
The men exposed to EST were expected to make similar kinds of judgements when it came to the female images.
But that isn't what happened. The results showed that exposure to AND or EST didn't affect results at all, suggesting that these two chemicals weren't influencing how attractive the participants found the opposite sex.
Of course, this is just one study, and it isn't enough to throw out all the previous research and evidence on AND and EST. But it does show that the research on these two chemicals isn't as clear-cut as much of the public has been lead to believe.
Simmons admits he still believes humans do have pheromones, but they're just not either of these two chemicals.
"I've convinced myself that AND and EST are not worth pursuing," he told Lindzi Wessel over at Science magazine.
But there are still many researchers who think AND and EST are pheromones.
Wen Zhou, a behavioural psychologist at the Chinese Academy of Sciences in Beijing, who wasn't involved in the research, told Wessel that the tape used to attach the chemical-soaked cotton balls to participants in this experiment could have messed with results.
"My major concern with the experiments in this study is that they were not rigorously designed and conducted," Zhou explained.
Martha McClintock, a behavioural neuroscientist at the University of Chicago in Illinois, who wasn't involved in this research and has been the driving force behind promoting AND and EST as pheromones, also told Science that the effect of the two chemicals could be more subtle than we're currently expecting.
"There's no doubt that this compound, even in tiny amounts, affects how the brain functions," she added.
So, the debate over human pheromones is far from over, but we now have some more evidence to throw into the mix that suggests AND and EST aren't the 'sex pheromones' they've been hyped up (and marketed by perfume manufacturers) to be.
Simmons hopes that future research will look into other possibilities, as well as continuing to investigate these two options.
"It shows the need for more studies in this area that are transparent and objective with the way they carry out the research, to help us deliver more conclusive results and find out if there are actually pheromones in humans," he said.
The study has been published in the journal Royal Society Open Science (note: the link is currently not working, it should be up in the next few hours).