Explaining the Decline of impal Marriage among the Karo: An Epidemiological Approach

The Karo of North Sumatra, Indonesia, have long expressed a preference for matrilateral cross-cousin (impal) marriage, yet the practice has declined sharply over the past century. To identify the demographic and sociocultural factors underlying this change, we applied an epidemiological approach to cultural evolution, using a case-control design coupled with structural equation modeling (SEM). Ethnographic data from 176 married individuals across 62 villages were analyzed to assess the effects of demographic, alliance, religious, cosocialisation, and modernisation variables on the likelihood of impal marriage. The model fit was strong (CFI = 0.96, RMSEA = 0.04), revealing significant direct and indirect effects. Marrying an impal was positively associated with the number of marriageable impal and with arranged or strategically motivated unions but negatively associated with reported religious conflict between spouses. Indicators of modernisation such as schooling and infrastructure were non-significant. These results indicate that the decline of impal marriage reflects demographic contraction and the weakening of alliance-based strategies rather than a rejection of the cultural norm itself. Framed within an epidemiological model of cultural transmission, the findings demonstrate how social practices may fade when the ecological and demographic conditions sustaining them erode, even as their normative value persists.