After throwing a scare into Sen. Ted Cruz (R-Tex.) in the 2018 midterm elections, Texas Democrat Beto O’Rourke is garnering attention as a possible 2020 presidential nominee. Political commentators are already arguing about O’Rourke’s potential strengths and weaknesses as a candidate, but he has at least one undeniable advantage: his height.

Voters like to think that they rationally assess political candidates entirely based upon policy positions, character and abilities. But more-primitive forces also shape political judgments, including a bias in favor of taller candidates. At a height of 6 feet 4 inches, O’Rourke stands (sorry, couldn’t resist) to benefit.

Since the beginning of the 20th century, no major political party has nominated a shorter-than-average man for president. (There are far, far too few data points to draw any conclusions about height and female nominees, but the one in all of U.S. history we’ve had so far is reportedly 5 feet 5 inches, slightly taller than average for women.)

And over the course of U.S. history, the taller presidential nominee has won the popular vote two-thirds of the time. Tallness bias is not unique to Americans. When asked to draw pictures of the “ideal national leader” and “typical citizen,” people from a range of countries draw the former as being taller than the latter.

Do taller candidates objectively deserve the credit voters give them? During childhood, height is positively associated with greater verbal ability and self-confidence, which are useful traits in a leader. But these characteristics vary less among presidential candidates than among the general population (presidential candidates almost by definition have high self-confidence), so it’s not likely they explain voters’ pro-tallness bias.

A preference for tallness in leaders is probably a more primitive human quirk bequeathed by evolution. Height is a proxy for strength and evolutionary fitness, and in most species taller and bigger creatures tend to win physical confrontations with shorter and smaller ones. Over millennia, human beings probably evolved a mental association between physical size and social dominance, including in the political realm.

All that said, being taller clearly doesn’t guarantee victory for a political candidate. Many comparatively short politicians have felled taller competitors. O’Rourke’s height-conscious political rivals might do well to seek advice from the Bush family. Former Florida governor Jeb Bush once stood on tiptoe in a photo of all the 2016 Republican primary presidential candidates. And 5-foot-11½ George W. Bush knocked off 6-foot-1 Al Gore (albeit while losing the popular vote) and 6-foot-4 John F. Kerry in consecutive elections.

But a distant relative of the Bush brothers, Franklin Pierce, was the greatest giant-killer in presidential political history. Facing off against the tallest major-party candidate in U.S. history — the 6-foot-5 Gen. Winfield Scott — the 5-10 Pierce cut him down to size to win the presidential election in 1852.