<p>The effects of birth order is an interesting topic in economics research. Most studies have used data from developed countries to show the adverse effects of birth order, demonstrating that individuals with higher birth orders are likelier to have lower educational attainment and earnings. Furthermore, studies have revealed adverse effects on other outcomes, such as health, intelligence, and intergenerational effects. However, the relationship of birth order with other household members, such as spouses, has not been investigated. Hence, this study assesses the association between the husband’s position in the birth order and their spouse’s “subjective well-being.” In particular, we examine how marrying the eldest son in a family relates to a woman’s happiness using data from the Japanese General Social Surveys. Our results reveal four findings. First, the wives of eldest sons are less happy than those of second or younger sons. Second, the lower happiness from marrying the oldest son is more pronounced for women in more recent birth cohorts (i.e., born in the 1970s and after) and those who marry the eldest son with no siblings. Third, married eldest sons are likelier to take responsibility for caring for their aging parents and help them with their housework. Fourth, unlike the wives of second or younger sons, wives of eldest sons show no difference in their preferences for work and parenting.</p>