It’s a great day to celebrate women’s movements past and present, but does feminism require the belief that women are basically like men?

This was at least an implicit claim of Second Wave feminists, modeled on prior movements for ethnic and racial equality. Blacks, Jews, and even the Irish had been targeted with biological theories. Their supposed inferiority was referred to “blood” in the nineteenth century, genes in the twentieth, but either way the goal was to “prove” that no education or cultural change would redeem them.

Anthropologists played an important role in proving the power of culture in group differences, showing that biology plays little or no part; culture easily accounts for ethnic variety in thinking, feeling, and behaving.

It made sense to think that gender would be similar: superficial biological differences with no implications for behavior. Proponents of male supremacy had claimed that women were held back not by men but by biology. For those struggling for gender equality, “Women can do anything men can do” was a clarion call.

All we have learned suggests that this was right: culture, upbringing, education, media, stereotypes, and men’s attempts to keep women down fully account for past differences in performance. Inequalities have shrunk as these influences have changed, and will likely continue to do so.

But what if gender biology is not as trivial as that of race? What if certain inclinations, affecting certain behaviors, are due in part to genes, hormones, and the brain? And what if in the end these differences could be viewed as favorable to women’s rights and opportunities?

This is neither a new nor an anti-feminist idea. Elizabeth Cady Stanton, in a speech before the National Woman Suffrage Convention in Washington, on January 19, 1869, said, “Those who represent what is called ‘the Woman’s Rights Movement,’ have argued their right to political equality from every standpoint of justice, religion, and logic, for the last twenty years.” She summarized them, and they are even more convincing now.

But she continued: “All these arguments we have to-day to offer for woman, and one, in addition, stronger than all besides, the difference in man and woman. Because man and woman are the complement of one another, we need woman’s thought in national affairs to make a safe and stable government.”

She offered an eloquent litany of male violence and oppression darkening the annals of history. We could give the same account almost word for word today. Stanton didn’t understand genes, hormones, or the brain—no one did—but she clearly believed some differences between men and women are what many academic feminists today would decry as essential.

She used that belief to advance her cause. “No one need wonder,” she said, “at the fragmentary condition of everything, when we remember that man, who represents but half a complete being, with but half an idea on every subject, has undertaken absolute control.” Stanton advocated a sixteenth amendment giving women the vote, but that had to wait half a century to become the nineteenth, passed by Congress on June 4, 1919 and fully ratified by the states on August 18, 1920.

Almost a century later, our leadership is not much more than half a being, but this is changing. We may elect a woman president—both sides have candidates—and the number of women has grown for decades in Congress and in corporate leadership, although it remains far too low.

Most sex differences are learned, but a few may indeed be essential. Men account for the vast majority of violence and exploitative sexuality in all cultures, and mounting evidence point to a role for genetic, hormonal, and brain differences. Meanwhile, social science research suggests that women lead, negotiate, and legislate more collaboratively and transparently than men.

We can’t yet link these differences to the biological ones. But research is confirming Stanton’s claim: Man is but half a being; we need women’s equal participation to make a whole world.