Some of the proposed mechanisms are: 
 Kin selection. That animals and humans are more altruistic towards close kin than to distant kin and non-kin has been confirmed in numerous studies across many different cultures. Even subtle cues indicating kinship may unconsciously increase altruistic behavior. One kinship cue is facial resemblance. One study found that slightly altering photographs to resemble the faces of study participants more closely increased the trust the participants expressed regarding depicted persons. Another cue is having the same family name, especially if rare, which has been found to increase helpful behavior. Another study found more cooperative behavior, the greater the number of perceived kin in a group. Using kinship terms in political speeches increased audience agreement with the speaker in one study. This effect was powerful for firstborns, who are typically close to their families.
 Vested interests. People are likely to suffer if their friends, allies and those from similar social ingroups suffer or disappear. Helping such group members may, therefore, also benefit the altruist. Making ingroup membership more noticeable increases cooperativeness. Extreme self-sacrifice towards the ingroup may be adaptive if a hostile outgroup threatens the entire ingroup.
 Reciprocal altruism. See also Reciprocity (evolution).
 Direct reciprocity. Research shows that it can be beneficial to help others if there is a chance that they will reciprocate the help. The effective tit for tat strategy is one game theoretic example. Many people seem to be following a similar strategy by cooperating if and only if others cooperate in return.
One consequence is that people are more cooperative with one another if they are more likely to interact again in the future. People tend to be less cooperative if they perceive that the frequency of helpers in the population is lower. They tend to help less if they see non-cooperativeness by others, and this effect tends to be stronger than the opposite effect of seeing cooperative behaviors. Simply changing the cooperative framing of a proposal may increase cooperativeness, such as calling it a "Community Game" instead of a "Wall Street Game".
A tendency towards reciprocity implies that people feel obligated to respond if someone helps them. This has been used by charities that give small gifts to potential donors hoping to induce reciprocity. Another method is to announce publicly that someone has given a large donation. The tendency to reciprocate can even generalize, so people become more helpful toward others after being helped. On the other hand, people will avoid or even retaliate against those perceived not to be cooperating. People sometimes mistakenly fail to help when they intended to, or their helping may not be noticed, which may cause unintended conflicts. As such, it may be an optimal strategy to be slightly forgiving of and have a slightly generous interpretation of non-cooperation.
People are more likely to cooperate on a task if they can communicate with one another first. This may be due to better cooperativeness assessments or promises exchange. They are more cooperative if they can gradually build trust instead of being asked to give extensive help immediately. Direct reciprocity and cooperation in a group can be increased by changing the focus and incentives from intra-group competition to larger-scale competitions, such as between groups or against the general population. Thus, giving grades and promotions based only on an individual's performance relative to a small local group, as is common, may reduce cooperative behaviors in the group.
 Indirect reciprocity. Because people avoid poor reciprocators and cheaters, a person's reputation is important. A person esteemed for their reciprocity is more likely to receive assistance, even from individuals they haven't directly interacted with before.
 Strong reciprocity. This form of reciprocity is expressed by people who invest more resources in cooperation and punishment than what is deemed optimal based on established theories of altruism.
 Pseudo-reciprocity. An organism behaves altruistically and the recipient does not reciprocate but has an increased chance of acting in a way that is selfish but also as a byproduct benefits the altruist.
 Costly signaling and the handicap principle. Altruism, by diverting resources from the altruist, can act as an "honest signal" of available resources and the skills to acquire them. This may signal to others that the altruist is a valuable potential partner. It may also signal interactive and cooperative intentions, since someone who does not expect to interact further in the future gains nothing from such costly signaling. While it's uncertain if costly signaling can predict long-term cooperative traits, people tend to trust helpers more. Costly signaling loses its value when everyone shares identical traits, resources, and cooperative intentions, but it gains significance as population variability in these aspects increases.