Many animals, including man, live socially. There are various kinds of cooperation and the importance of these outweighs the importance of any disruptive competition which occurs so the individuals stay together. Some social interaction is altruistic. An altruistic act by an individual is one which involves some cost to that individual in terms of reduced fitness but increases the fitness of one or more other individuals. Altruistic thoughts are those which involve an intention to carry out an altruistic act or to reaffirm the value of an altruistic act which is being or has been carried out. Reciprocal altruism occurs when an altruistic act by A directed towards B is followed by some equivalent act by B directed towards A, or by an act directed towards A whose occurrence is made more likely by the presence or behaviour of B. Some altruistic acts have a high actual or potential cost but still occur. Such acts may be reciprocated. In many societies there is care for the weak. These and other altruistic acts may be noticed by others and result in some benefit to the altruist or may pass unnoticed. Where reciprocation of altruism occurs there must be mechanisms for detecting cheats. Harmful acts and their consequences may also be noticed and have consequences. The consequences of various possible strategies in social situations can be modelled but the strategies considered have generally been too simple.