A new study sheds light on the yin and yang of the human animal — at once intensely competitive, at once intensely cooperative.

As one of the more social species on the planet, human beings interweave competition and cooperation in our interactions, with hormones promoting each — oxytocin and testosterone, among others — coursing simultaneously throughout the body.

Previous studies established that oxytocin promotes group bonding, diverting aggression toward others outside of the group. Now, Finnish researchers investigating human pheromones say men are influenced by androstadienone, a chemical component secreted through the sweat glands.

Earlier studies had shown that the hormone may function as a human pheromone, an airborne chemical intended to evince a response in other individuals. Although the pheromone phenomenon is largely unproven in the human species, studies suggest that androstadienone alters mood, cortisol levels, and activates brain areas linked to social cognition, perhaps drawing attention to the emitting person.

However, the evidence for androstadienone as a human pheromone has been lacking, particularly with regard to effects on men. Investigators from the University of Turku used behavioral game theory to test the effects of the purported pheromone.

Forty men were exposed to either the hormone or a control stimulus, then played a game intended to measure cooperation and generosity. One player would propose a way to share 10 Euros, and the other would either accept or reject. During the experiment, researchers measured salivary cortisol and testosterone, finding that cortisol levels roughly measured cooperative behavior of the men.

On average, the men exposed to androstadienone gave a half Euro, or 70 cents, more per transaction than the control group, and also accepted offers lower by that amount. Evolutionary biologist Markus Rantala, who led the study, expressed surprise however that men with higher levels of testosterone were also shown to be more generous. "I didn't know that this pheromone could interact with testosterone at all," Rantala told reporters.

Men with higher levels of testosterone, in addition to other male hormones promoting cooperation, may have won evolutionary success by building alliances with others, Rantala speculated. "[A]pparently such behavior is considered attractive by the opposite sex."

After accounting for individual variances in testosterone, androstadienone was associated with a boost in cooperative behavior, the researchers reported in the open-access journal PLOS ONE.

"Our study was the first to integrate the two distinct branches of research: human pheromone research and research on decision making behavior," the researchers write. "As such, it produced novel findings and, arguably, opens up interesting new possibilities for future research."

The research prompts the question on the relationship between androstadienone and sexual attraction and how the quantity the hormone may influence this effect, which researchers call the "chemical equivalent of a peacock's tail." With more evidence to support androstadienone as a human pheromone, scientists may better understand the connection between the hormonal stimulus and secondary behavioral responses evinced in others.

"Moreover, even if the signal is the same for both sexes, responses need not — and indeed should not — be," the researchers write. "An attractive and dominant male can be a valuable potential mating partner for a female, but precisely for the same reason, a competitor for another male."the

Also left for future study was the question of whether the effect of androstadienone as a pheromone differs by context, and whether production increases or decreases based upon situational outcome.

Source: Huoviala P, Rantala MJ. "A Putative Human Pheromone, Androstadienone, Increases Cooperation between Men." PLoS ONE. 2013.