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In 1990, Nobel-prize winning economist Amartya Sen stated that ‘more than 100 million women are missing.’ What exactly did he mean by ‘missing’ here? Girls become ‘missing’ because parents have a preference for sons in many countries around the world. Such preferences are often based on patriarchal and other unequal gender norms. Parents then regard boys as socially or economically more valuable. Paired with the arrival of modern technology to detect a child’s sex before birth, abortions of girls have led to highly unequal sex-ratios in countries such as China, India, and Azerbaijan.

The numbers are staggering. Take Armenia, for example. Reports indicate that from the 1990s onwards, after introducing ultrasound technology, the country saw a sharp shift in the sex ratio at birth, which peaked in 2000 with 120 boys born for every 100 girls. (Usually, girls and boys are born at a relatively stable rate of 105:100. The ratio approaches equality as they grow older because boys and men are more likely to die young.) Averaging across the years, there are almost 50.000 surplus males in a cohort of just above one million children and young adults.

What happens when there are much more men than women in a society? While the decision to abort a female fetus may seem rational on an individual level—for example, dowries increase the financial cost of having a daughter; often, sons provide more economic and physical security to their parents, since women tend to move away from their homes when they marry — many researchers suspect that the broader societal consequences of such gender imbalances are grim.

Men without women

Let’s first look at (young) males. In most societies, there are strong norms and pressures for them to find a partner and have children. Therefore, a generation of males born without an equally large number of women means that many males cannot adhere to these social norms. It may also affect their long-term economic prospect. In many countries without a developed welfare state, family structures (especially having children) are the only reliable institutions to provide for retirement.

While frustrations from being unable to find a partner may seem familiar to many people, an enduring and large shift in the ratio between males and females can become much more than an individual problem. In general, young men are often drawn to risky behavior (which also explains their higher mortality). But coupled with frustrations stemming from an overly male society, their actions become a risk factor for the entire country.

In China, for example, the One-Child Policy has had the unintended consequence of leading to a stark decline in female children. The result is a surplus of males (termed ‘bare branches’). Lena Edlund and coauthors report an increase in crime when these men came of age. A similar effect has been documented for India. And in an article published in the Journal of Conflict Resolution, Carlo Koos and I show that such inequalities can also cumulate in violence between ethnic groups in rural Africa. We assess how polygyny — the marriage between one man and several wives — leads to what we call ‘excess men’: men that cannot find a partner. Our results suggest that men in polygynous societies are, on average, more frustrated and ready to use violence.

A silver lining could be that the status of women increases due to their relative scarcity. Unfortunately, this does not seem to be the case.

But it would be a mistake to believe that these mechanisms are only at work outside Western Europe. While female feticide seems not to be a large issue here, we still observe sex-ratio shifts. For example, German women migrate from the Eastern federal states more often than men, leaving many rural communities with a surplus of 25 per cent men of marriable age. Some scholars think that this demographic development and the recent influx of immigrants created a poisonous mix, because immigrants who arrived within the past five years were young males to a larger degree. In a recent manuscript, Rafaela Dancygier and co-authors show that their arrival is connected to anti-immigrant hate crime in Germany. What is more, they show that half of the native men between 30 and 40 years in communities with many excess males express fear that they cannot find a partner due to immigration. Such fears of ‘mate competition’ are at the heart of the mechanism that drives the violent consequences of sex-ratio imbalances.

When women are missing, conflict follows

The violent consequences of unequal sex ratios may even transpire into international conflict. Take the recent escalation between Armenia and Azerbaijan. The latter, too, has experienced an uptick in male births. Therefore, both countries now have a young adult population marked by excess men. The discussed research findings from other contexts suggest that this could have been a factor contributing to a heightened readiness to employ violence. More generally, work by Valerie Hudson and coauthors has shown that the subordination of women goes hand in hand with less prosperity, fragile states, and conflict.

What about the women in male-dominated societies? A silver lining could be that the status of women increases due to their relative scarcity. Unfortunately, this does not seem to be the case. The frequently present patriarchal norms and the over-representation of males instead foster masculinisation and prevent female empowerment. Consequently, women tend to suffer, for instance, from sexual exploitation, (international) human trafficking and bride kidnapping.

Hence, missing women go hand in hand with the discrimination of women, amplifying a vicious cycle. Moreover, policies intended to support gender-equality can backfire. Sonia Bhalotra and co-authors show in a recent study that the introduction of gender-equal inheritance laws increased the economic costs of a daughter to their parents, due to the presence of customary gender and family norms. They document a decline in female births following the introduction of such a policy initially thought to empower women.

The unequal treatment of the genders has broad societal consequences beyond individual injustices. When women are ‘missing,’ violent conflict follows. Their absence is driven by norms that deny women equal rights and that often will be hard to change, as well as demographic developments that unfold over decades. The fact that such phenomena occur in both developed and developing countries suggests that researchers and policy-makers should pay attention to gender and family relationships not only as an arena for battles over equal treatment but as the basis for peace in societies.