This favoritism toward firstborn sons emerges before babies are even born; in fact, the India-Africa height gap is apparent at birth, and remains consistent through childhood. Families allocate inordinate resources — nutritious foods, iron supplements, tetanus shots and prenatal checkups — to a pregnant woman as long as there is a possibility that she is carrying the family’s firstborn son. Once a male heir is born, prenatal investments drop off.

In interviews we conducted in Gwalior this year, mothers freely admitted favoring their sons. And while they cited many reasons for investing less in their later pregnancies, including familiarity with the birthing process and dwindling income, their accounts strongly support the patterns we see in our data.

Interestingly, compared to their African counterparts, girls born before the family’s eldest son actually fare better than boys born after him, while girls born after the eldest son fare worst of all.

As these favored sons grow up, they continue to receive a disproportionate amount of their families’ resources. While firstborn Indian children receive, on average, one more essential childhood immunization than their counterparts in Africa, this is not the case for their younger siblings. Our interviews with mothers in Gwalior also found that younger siblings are fed less balanced diets.

India’s patrilineal traditions dictate that the eldest son care for his parents in old age and inherit property, while the dowries paid to marry off daughters can be expensive. The result is sex-selective abortion and an underinvestment in girls so common it has popularized a Hindi motto: “beti to bojh hoti hai,” meaning, “a daughter is a burden.” Recently, the cabinet minister Smriti Irani revealed that upon her birth, a friend of her mother’s quoted this maxim and urged her mother to kill her. It’s not surprising that in 2011, among Indian children age 6 and under, there were 919 girls for every 1,000 boys.

It is necessary and laudable to push for improving nutrition, advancing child and maternal health, and ending open defecation in India. However, we cannot close the malnutrition gap without addressing the social norms and economic rationales that deprive girls and younger siblings of the resources they need.

The fight for gender equality is a crucial part of this. Many recent policy efforts in India have focused on improving girls’ education; for these to have bite, they must be paired with efforts to equalize property ownership and job opportunities. A 2005 law granted daughters the right to inherit equally with their brothers, but enforcement is weak. And the participation of women in the labor force is actually declining in India today.

As long as this is the case, it will be hard to convince parents to invest in their children more equally. Parents will continue to lavish resources on their eldest sons if they see them as their only support in old age. But if we can improve the economic prospects for India’s women, we may be able to reduce the malnutrition of their children.