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Who’s the boss: A powerful congress means a healthier democracy.

When Kenya convulsed with violence after its flawed election in late December, many expressed surprise that one of Africa’s most stable countries could so quickly fall victim to ethnic hatred. But political scientists Steven Fish and Matthew Kroenig noted something else: a feeble legislature. Despite the opposition winning twice as many legislative seats as the president’s party, opposition members still took to the streets. Why? Because they wanted the only office that has any power in the country: the presidency.

In a groundbreaking new study, Fish and Kroenig rank the power of 158 national legislatures around the world, based on a survey completed by more than 700 country experts. The strength of parliaments and congresses is measured using four groups of factors: influence over the executive (such as powers of impeachment), autonomy (such as whether the executive can dissolve parliament), vested powers (such as the power to declare war), and the capability to get things done (such as having the resources to hire staff).

They find that countries with strong legislatures are far more likely to have resilient democracies. Weak legislatures often cannot keep executives in check, especially when autocratic leaders come to power. “This decade, the great enemies of democracy are presidents,” says Fish, a professor at the University of California, Berkeley. “If you have a legislature that’s vested with the...