A new study has said that oxytocin, the so-called love hormone, which fosters the bond between mothers and kids, can instil both self-sacrifice and defensive aggression among warriors.
Researchers have claimed that oxytocin can goad soldiers to launch preemptive strikes in defence of their comrades.
Oxytocin has received much attention for boosting social bonding and cooperation, but it also appears to trigger defensive aggression against outsiders who might threaten an individual's social group, psychologists say.
This indicates that the hormone has a much more complex role in social dynamics than just encouraging humans to make love and not war.
"Our study shows that oxytocin not only plays a role in modulating cooperation and benevolence, but also in driving aggression," Live Science quoted Carsten De Dreu, a social psychologist at the University of Amsterdam in the Netherlands, as saying.
Dreu and colleagues have pointed towards oxytocin as a likely neurobiological mechanism that drives how humans regulate intergroup conflict.
Some animal studies had shown that oxytocin encourages protectionist behaviour, but this marks the first study to illustrate a similar effect in humans.
The researchers had reasoned that this "dark side" of cooperation makes sense from an adaptive, evolutionary perspective of competing groups.
"We were interested in seeing where oxytocin's 'niceness' breaks down," said De Dreu.
To study the dark side of oxytocin, the researchers ran three experiments based on financial games that represented variations on the classic prisoner's dilemma scenario.
The games pitted self-interest against the overall interest of each three-person group, and also added the possibility of hurting a competing three-person group.
Individuals could either keep a certain sum of money or put it into a group pool in which the individual got less but the whole group benefited more.
Male volunteers who took a whiff of oxytocin through a nasal spray tended to act more in the interests of the group (sharing their money) rather than selfishly, unsurprisingly.
They also tended to make choices that benefited their group but did not hurt outsiders during the first experiment.
The second experiment showed that oxytocin affected people regardless of their natural tendencies to cooperate.
But the real twist came during the third experiment involving 79 males, who took either oxytocin or a placebo.
Instead of having a certain amount of money to spend, the group decision-makers simply chose whether to cooperate or not cooperate with an outsider group.
It was observed that decision-makers under the influence of oxytocin acted protectively by not cooperating with an opposing group, as researchers had predicted.
Such noncooperation in the third experiment was considered a preemptive strike or defensive aggression, because the group acted to protect itself against possible harm from the outsiders.
The third experiment also showed that oxytocin encouraged defensive aggression against outsider groups when there was greater fear of such groups, explained De Dreu.
The results showed that oxytocin did not encourage such offensive aggression, in which a group would "hurt" another group without having been provoked, aimed only at winning more rewards.
Researchers cautioned that the findings only apply to males so far, given that no females participated in the experiments.
The researchers found that giving oxytocin to everyone in the world won't necessarily usher in a new era of peace and prosperity.
It might even spur more paranoia and conflict between different groups or nations.
"Giving soldiers oxytocin might make them more cooperative towards their comrades, even willing to self-sacrifice. But it should [also] make them more likely to launch a preemptive strike against the competing army, with conflict-escalation being the most likely consequence," said De Dreu.
The study was published in the journal Science.