In 1891, officials of the British South Africa Company drew a line on a map in order to carve out two new districts in the lands under their control, heedlessly slicing through the traditional boundaries of the Chewa and Tumbuka tribes. It’s the kind of story that’s been repeated many times in Africa and elsewhere, with arbitrary boundaries tragically setting the stage for future tribal and ethnic conflict. But in this case, there is a difference. On one side of the border, in what is now Malawi, Chewa and Tumbuka today are at war culturally and politically, just as one would predict. In neighboring Zambia, however, the two tribes are allies and “brethren.”

Why is this so? It’s not that cultural differences are more pronounced in Malawi, according to Posner, a political scientist at the University of California, Los Angeles. In surveys, he found that members of the two tribes on each side of the border point to the same basic divisions: Tumbuka parents, for example, demand a large price of perhaps seven cows when their daughters marry, while Chewa parents are happy with a single chicken. But in Malawi, people are more likely to attach negatives to their descriptions: Tumbukas call the Chewas “lazy,” and Chewas return the favor by calling the Tumbukas conceited.

The explanation for the cross-border difference, Posner argues, is that in Zambia both tribes make up too small a part of the national population (less than seven percent each) to form a distinctive group or, more important, a bloc big enough for political leaders to exploit. In Malawi, however, the Chewa are 28 percent of the total, the Tumbuka 12 percent.

The coming of democracy crystallized the national differences. In 1994, when Malawians finally got a chance to vote, in the country’s first election, longtime dictator Hastings Kamuzu Banda played the tribal card with a vengeance, warning his fellow Chewas of Tumbuka threats to their interests and exacerbating ethnic tensions in the process. In Zambia, which held its first multiparty elections in 1991, President Kenneth Kaunda appealed to Chewas and Tumbukas not as separate groups but as “Easterners” who needed to unite in order to defeat their rivals elsewhere in the country. They gave him more than three-quarters of their votes.

Posner suggests that this “natural experiment” in Africa could shed light on supposedly culture-based conflicts in other parts of the world. And while it’s too late to redraw national boundaries, some of his research suggests that shrewdly drawn regional boundaries within nations might produce some of the results British colonialists accidentally achieved in Zambia.