1 Child
A study headed by Hans-Peter Kohler compared happiness levels in adult identical twins — some of whom are parents and some who aren’t.
Kohler found that mothers with one child are about 20% happier than their childless counterparts. Fathers’ happiness gains are smaller, and men enjoy an almost 75% larger happiness boost from a firstborn son than from a firstborn daughter. (The first child’s sex doesn’t matter to mothers.)
Second and third children don’t add to parents’ happiness. They make mothers less happy than mothers with only one child — though still happier than women with no children.