An accepted, established, or expected pattern of behaviour. Social customs guide behaviour and can replace the need to make choices. For example, it is a social custom in many countries that a man should open a door for a woman. When two people meet at a door this custom solves the decision problem of who should open it approximately half of the time. The concept of social custom has been used to explain observations of behaviour that are otherwise not individually rational. Examples include the membership of a trade union when non-members also benefit from the gains secured by the union, and the decision not to evade tax even when the expected return from evasion is positive.