Kohlberg emphasized that human behavior becomes less selfish as we mature due to:

Kohlberg's theory of moral development outlines how individuals progress through stages of moral reasoning as they mature. He posited that as people develop socially, they begin to understand and consider the perspectives, needs, and rights of others, leading to less selfish behavior. This social development involves engaging with and recognizing the complexities of societal norms and ethical considerations, which helps individuals move from self-centered reasoning to a more principled understanding of morality.

The choices related to physical, cognitive, and economic development do not encompass the social aspects that are central to Kohlberg's theory. Physical development pertains more to growth and bodily changes, cognitive development relates to the growth of thought processes but does not specifically address moral reasoning in the social context, and economic development does not directly influence individual moral reasoning as outlined by Kohlberg. Thus, social development is the most relevant factor in explaining the maturation of moral reasoning and the decline in selfish behavior as individuals grow.