For generations, Karo people in North Sumatra, Indonesia, have expressed a preference for marriage to a particular type of cousin, their impal, or matrilateral cross-cousin. Yet these marriages have declined dramatically.
In the 1930s-1960s, around 11-13% of marriages were between genealogically defined impal. By the 1990s-2014, this had fallen to just 3-5%.
Did Karo people simply stop believing in impal marriage?
Our new study, forthcoming in Evolutionary Human Sciences, suggests something more interesting. The cultural ideal can persist even when the behaviour becomes increasingly difficult to practise.
We interviewed 176 married Karo people from 62 villages, comparing those who had married an impal with those who had not. Using structural equation modelling, we examined how demographic, religious, social, and economic factors shaped marriage decisions.
The strongest finding was surprisingly straightforward. Opportunity matters. People were substantially more likely to marry an impal when they had more marriageable impal available. As families become smaller, there are simply fewer potential cousins to marry.
Economic changes matter too. Impal marriages were more likely to be arranged and strategically motivated, particularly where property and land were involved. Land fragmentation and alliance-related factors have weakened the incentives.
Surprisingly, religion, education, and modern transportation infrastructure were not directly associated with impal marriage. This challenges the simple idea that religion or “modernisation” causes traditional practices to disappear.
The broader lesson extends beyond Karo marriage. Cultural practices do not necessarily disappear because people reject them. Sometimes the social and ecological conditions that make them possible disappear first.
In this case, the norm may be fading not because Karo people stopped valuing impal marriage, but because the world in which it made sense – and was possible – has changed.
