It is not true that my book “Guns, Germs and Steel,” as Mr. Romney described it in a speech in Jerusalem, “basically says the physical characteristics of the land account for the differences in the success of the people that live there. There is iron ore on the land and so forth.”

That is so different from what my book actually says that I have to doubt whether Mr. Romney read it. My focus was mostly on biological features, like plant and animal species, and among physical characteristics, the ones I mentioned were continents’ sizes and shapes and relative isolation. I said nothing about iron ore, which is so widespread that its distribution has had little effect on the different successes of different peoples. (As I learned this week, Mr. Romney also mischaracterized my book in his memoir, “No Apology: Believe in America.”)…

Just as a happy marriage depends on many different factors, so do national wealth and power. That is not to deny culture’s significance. Some countries have political institutions and cultural practices — honest government, rule of law, opportunities to accumulate money — that reward hard work. Others don’t. Familiar examples are the contrasts between neighboring countries sharing similar environments but with very different institutions. (Think of South Korea versus North Korea, or Haiti versus the Dominican Republic.) Rich, powerful countries tend to have good institutions that reward hard work. But institutions and culture aren’t the whole answer, because some countries notorious for bad institutions (like Italy and Argentina) are rich, while some virtuous countries (like Tanzania and Bhutan) are poor.