Reciprocal altruism is helping with the expectation that the other will help back later; Robert Trivers developed the idea to explain cooperation. Tit for tat copies what the other player did last, starts with cooperation, and won two competitions against other strategies. In the public goods game, each player secretly chooses how many private tokens to put into a shared pot, which is used to study altruism and cooperation. In repeated public goods games, giving typically declines over rounds but rarely drops to zero. In the dictator game one person decides how much to give to another, who has no say, so it measures how much a person is willing to give. In a brain-scan study, activity was larger when people gave money voluntarily than when they paid it as taxes.

Wilkinson first reported in 1984 that vampire bats may share regurgitated blood with non-relatives. Vampire bats form strong bonds with their colony mates, and some unrelated bats have traveled together for more than ten years. A vampire bat can share some of its blood meal with another bat in its colony. A vampire bat that found no food can beg another bat for blood, and a small amount lets it live and hunt another day. In captive vampire bats, food sharing was rare and was always preceded by grooming, a lower-cost investment; the researchers called this support for the raising-the-stakes idea. In captive bats, those that were more generous were fed more when they failed to feed. Vampire bats may not yet qualify as an unequivocal example of reciprocal altruism.

A free rat will open the door of a trap to let out a rat from its own cage that is stuck inside. The researchers did not train the rats to free their cagemates. A rat was as likely to free its trapped cagemate first as to open a container of chocolate. A free rat acted more upset when its cagemate was trapped, as if it picked up the cagemate's distress. Alarm calls were rare in the rat study, so the authors judged that the free rat was unlikely to be opening the door just to end the alarm calls. Rats freed trapped cagemates even when the reward of social contact was prevented, so the contact was not the only reason.

In one task, an adult hung clothes on a line and dropped a clothes peg that he could not reach. Toddlers who were 18 months old helped a grown-up they had never met. In the 2006 toddler study, virtually all of the children helped at least once, and the adult never asked them for help. Helping behavior is seen in humans from about two years old, when a toddler can understand subtle emotional cues. Warneken argued that people have a biological predisposition to be helpful, so not much moral instruction is needed to develop helping. Young chimpanzees helped a person who was reaching for something. Nobody asked or rewarded the toddlers and chimpanzees for helping.

In a study of chimpanzees, females were more consoling than males overall, and bystanders seemed sensitive to the neglected victims' need for comfort. Asian elephants comfort other elephants that are upset, using touch and sounds. An elephant may put its trunk in another elephant's mouth to calm it, a gesture a researcher compared to a handshake or hug. As of November 2026, a 2014 study gave the first scientific evidence that elephants console one another. In a study, pet dogs went up to a person who was crying. Dogs in a study went to a crying stranger, not only to their owner. The dog study used a method first used with human infants, and most dogs responded to the crying person in a submissive way.

Researchers use several terms for how people respond to the feelings of others. Empathy is sensing and sharing what another person is feeling. Emotional contagion is the spontaneous spread of emotions between people. Mimicry is thought to be one foundation of how emotion moves between people. Cognitive empathy means understanding another's perspective or mental state, and it is a different thing from affective empathy. The study authors wrote that separating your own feelings from another's is crucial for acting on empathic concern. Having empathy does not always mean a person will want to help someone in need. The Interpersonal Reactivity Index is a 28-item self-report test by Mark Davis that measures empathy.

Mirror neurons were first found in macaque monkeys: some cells fired when a monkey grasped food and when it watched a person do the same. In macaques, mirror neurons are found in the inferior frontal gyrus (area F5) and the inferior parietal lobule. One analysis found no relationship between motor empathy and mirror neuron activity. Scientists still argue about what mirror neurons do in people, including how much they explain reading other minds. Oxytocin is a peptide hormone and neuropeptide normally made in the hypothalamus and released by the posterior pituitary. Oxytocin has a role in social behaviors in many species, so it likely also does in humans, and it may play a part in bonding between humans and dogs. Studies of nasal oxytocin have been criticized: evidence suggests nasal sprays may not produce behavioral effects, and results may reflect publication bias.

Doing something kind for another person can make you feel happier. A psychologist's study found that doing kind acts raised people's well-being. Doing a variety of kind acts seems to work better than repeating the same one. In a study of children under two, toddlers seemed happier giving a treat away than getting one. In a bonus study, how workers spent the money, not how big the bonus was, changed their happiness. In one experiment, people were handed a $5 or $20 bill and told to spend it by 5 p.m. that day; those who spent it on others reported more happiness. Regular volunteering has been linked to a lower risk of dying and better physical functioning into old age.

In one experiment, one person's generosity spread to three people and then to nine. In experiments, generous behavior spread through a group, and so did stingy behavior. In a cooperation experiment, a small number of generous people changed how other people in the group behaved. Christakis compared a social network to a matching grant: one person's giving in the first round was ultimately tripled by others. Cooperative behavior in the experiments spread up to three degrees of separation. A science writer noted that Fowler and Christakis report that both cooperative and selfish acts spread, so it is unclear how the ripples affect real networks.

Words to know

• reciprocal: back and forth
• mimicry: copying
• oxytocin: a hormone
• altruism: selfless concern
• consoling: comforting