A striking new study from a trio of European economists has found that mothers whose first-born child is a girl tend to work significantly more hours than mothers whose first child is a boy — a result that holds across multiple countries and income levels, and that has economists rethinking assumptions about how children affect women's careers.

The finding runs counter to conventional wisdom. Conventional economic models predict that the effect of children on maternal labor supply should depend primarily on the number and ages of children, not their sex. But the data tell a different story.

The researchers' leading explanation involves parenting norms and perceived developmental needs. Daughters, studies consistently show, are more likely to be involved in household chores and childcare for younger siblings from an early age. Mothers with older daughters may benefit from this unpaid labor in ways that allow them to return to or increase paid work. The effect is small but measurable and persistent.

There is also a psychological dimension. Some researchers have pointed to evidence that parents — and particularly mothers — invest more anxious attention in boys, spending more direct supervision time with sons and allowing daughters more independence at earlier ages. If that pattern holds, it would reduce the time burden on mothers of girls relative to mothers of boys of comparable ages.

The implications are modest but interesting. They suggest that the relationship between children and maternal labor supply is more nuanced than economists have typically modeled, and that even the sex composition of a family can have measurable effects on how mothers allocate their time between paid and unpaid work. For policymakers thinking about family leave, childcare, and gender gaps in the labor market, the details matter.