How Your Brain Determines If You're a Santa or a Scrooge

They say there's something about this time of year that just puts people in the giving spirit. But more than twinkling lights and sappy music, it's likely that surge of hormones rushing through your brain are what make you feel generous, according to a new study published Monday in Proceedings of the National Academy of Sciences.

The amygdala (a region of the brain long associated with fear instincts) and the hormone oxytocin (which aids intimacy and social bonding) may play a crucial role when it comes to deciding whether or not to lend a hand to others, according to the paper, which tested the theory on rhesus macaque monkeys. And it's likely a similar process is happening in the human brain, meaning an abundance of oxytocin in your amygdala could make you a more generous person.

Researchers trained the macaques to associate different color-coded shapes on a screen with four different outcomes: a serving of juice for themselves, juice for another monkey only, juice for both of them, or juice for nobody. Once the monkeys were well-versed in the coded shapes, they were presented with two scenarios. In the first scenario, the "actor" monkey (the one making the decision) could choose to either have juice just for itself, or for both itself and the "recipient" monkey. In the second scenario, the actor could choose to give juice only to the other monkey or for nobody to get any juice.

"From the actor's perspective, there's no benefit or harm from choosing one or the other, because he always gets the same amount of juice," said Steve Chang, a professor of psychology at Yale and lead author of the study. "So any choice it makes is a socially-motivated choice."

In the first scenario the actors preferred to be selfish and more often chose to just have juice for themselves. But in the second scenario, the actor was more likely to choose for the other monkey to have some juice than for nobody to get anything at all.

These choices were interesting in their own right, but the researchers were more curious about what was happening in the monkeys' brains while these decisions were being made. They found that certain neurons in the amygdala were activated when the monkeys were evaluating the choice of rewards, and that the same value-assessing neurons were lit up whether monkey chose to be selfish (like in the first scenario) or generous (as in the second scenario). Chang said this suggests the monkeys weigh a rewards value to themselves the same way they weigh its value to others.

"For example, if a chocolate candy is really valuable to me, these neurons would cause me to consider chocolate candy to be very valuable to another person as well," Chang explained.

The researchers wondered if these neurons could be firing just by experiencing the reward being delivered—if they were tied to the action of getting or giving juice, rather than the decision beforehand. To determine if this was the case, they had the monkeys participate in trials where they didn't have any agency: whether or not they got juice or gave juice was predetermined. During these trials, they found the neurons weren't notably active, making it much more likely the brain processes were linked to the decisions themselves.

To go a little deeper, the researchers also tried injecting a small amount of oxytocin—which has been previously linked to enhanced prosocial behaviors—directly into the amygdala and, as a control, another part of the brain, to see if it would affect the monkeys' choices. They found only when oxytocin was injected into the amygdala, there was about a 20 percent increase in actor monkeys choosing to give juice to recipient monkeys, even in the "just me or both" scenarios.

"And on those trials when the monkeys were very generous, they also tended to look at the other monkey more after the juice was delivered," Chang said. "Even a tiny bit of oxytocin in the amygdala changed the social behavior, which suggests the amygdala itself is causally involved in this kind of decision making."

Chang said it's fair to hypothesize that a similar function happens in the human brain when deciding whether or not to be generous because macaques are very similar to humans when it comes to complex social behaviors. It also means that if a monkey or a person has an abundance of oxytocin flowing through his or her amygdala, they're likely more generous. Previous studies have found monkeys reared with their mothers, as opposed to in a nursery, developed naturally higher levels of oxytocin, and also were more social as adults, which Chang says makes sense with the findings he and his colleagues published.

"Causally it's harder to know because there hasn't been larger scale causal studies," Chang said. "But there have been suggestions that the environments that reduce oxytocin processing are likely to decrease prosocial behavior."

Like all brain science, it's more complicated than a simple A causes B scenario, but the findings make it likely that the generous feeling in your heart in early winter has more to do with your monkey brain than you realize.