This week, the United States will celebrate 100 years of women’s suffrage. The Nineteenth Amendment, ratified on August 18, 1920, and certified as part of the Constitution on August 26, 1920, granted women the right to vote. In practice, many women—particularly Black women in the South—could not exercise their right to vote until decades later. Still, women’s suffrage altered the political landscape in the United States. And as it spread across the democratic world in the first half of the twentieth century, women’s suffrage altered international politics, as well—in particular, by reducing the likelihood of interstate conflict, especially between democratic countries.  

That democracies are unlikely to fight one another is one of the few widely accepted findings in international relations. Democracies do fight with nondemocracies, but they are less likely than nondemocracies to initiate such conflicts. What explains this “democratic peace” is still a matter of debate. Is it democratic norms? Democratic institutions? International trade? Or some combination of these factors? In new research to be published this month in the journal International Organization, we show that women’s suffrage is an important contributor to the democratic peace. Women are less likely than men to support the use of military force. Their pacific preferences don’t always prevail within democratic institutions, but our research shows that democratic states with women’s suffrage initiate fewer disputes with all countries and that they experience more peaceful relations with other suffrage democracies.


The pacifying effect of women’s suffrage ultimately stems from the preferences of individual female voters. As the political scientist Joshua Goldstein outlined in War and Gender, there is abundant evidence that women are less inclined toward violence than men, across time periods and national contexts. American women (and although data are sparser, women of other nationalities) generally express higher levels of support for peaceful actions than do men in public opinion surveys. Polls likewise show consistently lower levels of support for military action among American women compared with American men, although the size of the gender gap varies over time and with context. In the United States before World War II, for instance, support for aiding the United Kingdom, even at the risk of getting into a war with Germany, was about ten percentage points higher among men than among women. In recent years, the gender gap on the approval of U.S. drone strikes has been about 17 percentage points.

Our study took this gender gap in public opinion on the use of force as a starting point. But instead of relying on polls, which typically ask respondents to consider just one scenario (for example, whether they support a war that has already started, not whether they supported the war in the first place), we examined survey experiments that allow researchers to randomly assign respondents to different scenarios and thereby achieve a fuller picture of their preferences. We conducted a meta-analysis of recent survey experiments by international relations researchers who study military conflict, comparing scenarios that involved the use of military force with at least one alternative where force was not used or where it was used conditionally. In total, we examined 17 previous studies and conducted four original experiments, spanning six countries (Egypt, Israel, Japan, Turkey, the United Kingdom, and the United States) and encompassing more than 20,000 total respondents. The studies examined many different reasons for using force, including conflict over resources, humanitarian intervention, preemptive war to stop a country from acquiring nuclear weapons, and intervention to protect an ally.

There is abundant evidence that women are less inclined toward violence than men, across time periods and national contexts.

In all the studies we analyzed, men were more inclined to use force than were women. Across all studies, the average level of support among men for using force was just over 50 percent, compared with 38 percent for women. The size of the gender gap varied—for example, the gap was largest in Japan and smallest in Israel, perhaps reflecting the particular role of the military in those countries—but it existed in all the countries and settings we examined.

The numbers do not suggest that women are pacifists. Nearly 40 percent of women supported the use of force, on average, and in two of the studies we considered, women’s support for the use of force exceeded 50 percent. But women’s hawkishness depended on context and could change with new information—often moving in parallel with changes in men’s views of the use of force. For example, in August 1940, 52 percent of U.S. men and 39 percent of U.S. women supported taking actions that risked war with Germany. A month later, after Germany began systematically bombing London, approval among men and women had each increased by seven percentage points. And a month after that, male approval had increased by another seven percentage points and female approval by another nine percentage points, meaning that clear majorities of both sexes favored taking actions that risked war. These findings are consistent with prior public opinion research. As the political scientist Rose McDermott has argued, men and women often fight for different reasons and under different circumstances, with women more focused than men on protecting home territory and less concerned with status competition. But the key finding that our research confirms is that women have lower baseline levels of support for the use of violence and tend to view belligerence more negatively than men do.


Popular preferences—even widely shared ones—don’t always translate into national policy. But in democratic countries, voter preferences shape the behavior of democratic institutions—so as the quality of democracy improves, women’s suffrage should have a greater impact on matters of war and peace. Given that women are less likely than men to support aggressive international action, one would expect democracies with women’s suffrage to initiate fewer conflicts than democracies without women’s suffrage (and nondemocracies with women’s suffrage—yes, there are some of those), regardless of whether the potential adversary is a democracy or not. Likewise, if a dispute breaks out between two democracies with women’s suffrage, the likelihood of militarized conflict should be lower than if the dispute is between nonsuffrage democracies.

To test these hypotheses, we gathered data on the voting status of women by year in 196 countries between 1816 and 2010—a period that enables before-and-after comparisons in democratic countries that granted women the right to vote between 1893 (New Zealand) and 1971 (Switzerland). Our findings indicate that two democracies with women’s suffrage were roughly one-third as likely to engage in a militarized dispute as two democracies without women’s suffrage and about one-fourth as likely as two autocratic countries. Democracies without women’s suffrage were 192 percent more likely to initiate conflicts than democracies with women’s suffrage. Autocracies were 163 percent more likely. When looking exclusively at the pre–World War II era, we found similar results, suggesting that the economic interdependence, international institutions, and nuclear weapons that define the postwar era are unlikely to explain our findings.  

Of course, there are other potential causes of these results. Many democracies—including the United States and the United Kingdom—extended suffrage to women in the aftermath of World War I. Perhaps war weariness or more cooperative international norms in the interwar period led to women’s suffrage, and not the other way around. But our statistical analysis suggests that the tendency of women’s suffrage to promote peace is not confined to a particular historical time period. Our findings are consistent with those of other scholars who have shown that the extension of suffrage did not come about as a direct result of World War I. Indeed, in the United States, many states already had women’s suffrage before World War I (and thus before the Nineteenth Amendment was ratified).  

Women’s suffrage didn’t spell the end of interstate conflict—it didn’t stop World War II, nor did it prevent many of the hot conflicts of the Cold War. But extending the vote to women may have averted conflicts that otherwise would have broken out. Just as the Nineteenth Amendment was an incomplete extension of rights—with Black women continuing to face major obstacles to voting for many decades—the extension of women’s suffrage was an incomplete but important pacifying force in international politics. 

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  • JOSLYN N. BARNHART is Assistant Professor of Political Science at the University of California, Santa Barbara.
  • ROBERT F. TRAGER is Associate Professor of Political Science at the University of California, Los Angeles.
  • ELIZABETH N. SAUNDERS is an Associate Professor in the Security Studies Program at the School of Foreign Service at Georgetown University.
  • ALLAN DAFOE is an Associate Professor and Director of the Centre for the Governance of AI at the Future of Humanity Institute, University of Oxford.
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