Why is the movement of population in the Americas almost wholly one-way? I pondered the question this week as I traveled from Florida to Guatemala and back. No Mexican politician has ever had to suggest building a wall to keep the gringos out. Around 160,000 Mexicans settle legally in the United States each year (it is, by definition, impossible to count the illegal ones), yet hardly any Americans make the reverse journey. Why?

On one level, the answer is obvious. Incomes in the U.S. are around seven times higher than in Mexico, and around 10 times higher than in most Central American countries. Americans take for granted things that are luxuries for many of their southern neighbors: clean water, steady electricity, decent schools, quality healthcare, noncorrupt judges, regular elections.

But why do Americans take these things for granted? Is it because of some magical property in the water? Or perhaps in the gene pool? Hardly. In large parts of Texas, New Mexico, and Arizona, the populations immediately on either side of the border are pretty much identical. They breathe the same air, eat the same food, watch the same television. Yet the U.S. citizens are vastly better off than their sundered kindred.

Alexis de Tocqueville glimpsed the explanation 180 years ago. He argued that the societies of the Americas were primarily defined by what he called their “point de départ,” the places where they had originated. In the wide spaces of the New World, he believed, people intensified the tendencies they had carried with them from the Old. Thus, French America exaggerated the seigneurialism and obscurantism of Bourbon France, and Spanish America the ramshackle corruption of Bourbon Spain. But English America (as Tocqueville always called it) exaggerated the individualism and localism of parliamentary England.

“We have no reason to believe that, at the beginning of the sixteenth century, the Castilian was in any respect inferior to the Englishman,” observed Tocqueville’s British contemporary, the poet and historian Lord Macaulay. The southern continent had greater resources than the northern — not least the precious metals that were soon weighing down the hulls of Spanish galleons.

But the English-speaking settlers brought with them a treasure more precious than all the silver in Potosí. They had a way of ordering their affairs that was uniquely suited to building an open society.

The Spanish empire was a nationalized enterprise, and the Spanish monarchy, like all autocratic institutions, was jealous of its power. In 1767, the Jesuits, who had been leading the missionary work in the Americas, were accused of “building an empire within an empire” and repressed.

North America, by contrast, was largely left to its own devices, a policy characterized by Edmund Burke as “wise and salutary neglect.” Where Spain sought to run its empire from royal palaces, Britain allowed its colonists to elect their own legislatures, make their own laws, and, at least for the first century-and-a-half, set their own taxes.

The Spanish conquerors sought to replicate the vast agrarian estates of their native Extremadura and Andalusia, imposing a form of serfdom on the indigenous peoples similar to that which had been imposed on the conquered Moors a century earlier. A system known as the encomienda divided the natives into groups who were obliged to offer forced labor to their Spanish masters, the encomendneros. Power and money were hoarded in the hands of a small elite. Wealth came primarily from predation rather than production.

In North America, things were very different. In 1618, the Virginia Company offered every settler 50 acres, plus 50 more for every family member who joined him, an allocation known as the “headright” that was to set the pattern for subsequent Homestead Acts until the entire continent had been filled. This rapid expansion was catastrophic for the indigenous peoples, to whom most settlers were shockingly indifferent. It did, however, create a rules-based and soon-democratic society.

Now, here’s the point: It is North America that is the outlier. The system that the Spanish imposed on Latin America was similar to the preexisting social orders of the Aztecs and the Incas — and, indeed, of almost every other civilization for the past 10,000 years. A small number of people would rig the rules so that they and their children could prey on everyone else.

It is possible, of course, for other countries to share in the benefits of liberal democracy by copying the structures of the Anglosphere. Over the years, it has happened in several places. But isn’t it just as possible for the U.S. to revert to the mean? Indeed, given its exceptional nature, and given that nature tends to entropy, isn’t that the likelier outcome?