Altruism is a voluntary action aiming to help others without reward expectation. In this type of action, the individual cares for others' interests rather than those of his own. This type of behaviour that goes beyond social norms falls into the sphere of morality. The frequency of such actions in society promises ethical behaviour. In this regard, the role of the family as one of the most important agents of socialisation is highlighted. This article seeks to examine altruism among the Iranian families and show its process of change over a decade (from 2005 to 2015). The findings of a longitudinal study were used to achieve this objective. This survey was done in 2005, for the first time, and was repeated in 2015, for the second time, in two developed and less developed regions of Tehran, using cluster sampling. The results show that the number of people showing altruistic actions declined by 7percent between 2005 and 2015 (from 26 percent to 19.2 percent). The number of people having high altruistic attitudes was higher in 2005 compared to 2015 (90.9 percent versus 86.6 percent). Also, the number of people with high cultural capital was more in 2005 compared to 2015. Accordingly, altruism among Tehranian citizens has declined within a decade, while the economic and social capital has slightly increased. In both surveys, there is a noteworthy relationship between social capital and altruistic attitudes with altruistic actions, specifically regarding marital status.