Linguistic Exogamy Is A Thing

Who knew that there were cultures in which it was mandatory to marry someone who didn't speak the same language that you did?

I guess it provides an excuse for the marital miscommunications which are inevitable in any case, and encourages understanding of them. It could also promote language learning necessary for effective regional ties.

Human populations often exhibit contrasting patterns of genetic diversity in the mtDNA and the non-recombining portion of the Y-chromosome (NRY), which reflect sex-specific cultural behaviors and population histories. 

Here, we sequenced 2.3 Mb of the NRY from 284 individuals representing more than 30 Native-American groups from Northwestern Amazonia (NWA) and compared these data to previously generated mtDNA genomes from the same groups, to investigate the impact of cultural practices on genetic diversity and gain new insights about NWA population history. Relevant cultural practices in NWA include postmarital residential rules and linguistic-exogamy, a marital practice in which men are required to marry women speaking a different language. 

We identified 2,969 SNPs in the NRY sequences; only 925 SNPs were previously described. The NRY and mtDNA data showed that males and females experienced different demographic histories: the female effective population size has been larger than that of males through time, and both markers show an increase in lineage diversification beginning ~5,000 years ago, with a male-specific expansion occurring ~3,500 years ago. 

These dates are too recent to be associated with agriculture, therefore we propose that they reflect technological innovations and the expansion of regional trade networks documented in the archaeological evidence. Furthermore, our study provides evidence of the impact of postmarital residence rules and linguistic exogamy on genetic diversity patterns. Finally, we highlight the importance of analyzing high-resolution mtDNA and NRY sequences to reconstruct demographic history, since this can differ considerably between males and females.

Leonardo Arias, et al., "Cultural Innovations influence patterns of genetic diversity in Northwestern Amazonia" BioRxiv (June 14, 2018) doi: https://doi.org/10.1101/347336

See also Luke Fleming, "Linguistic exogamy and language shift in the northwest Amazon" 240 International Journal of the Sociology of Language (May 5, 2016) https://doi.org/10.1515/ijsl-2016-0013

The sociocultural complex of the northwest Amazon is remarkable for its system of linguistic exogamy in which individuals marry outside their language groups. This article illustrates how linguistic exogamy crucially relies upon the alignment of descent and post-marital residence. Native ideologies apprehend languages as the inalienable possessions of patrilineally reckoned descent groups. At the same time, post-marital residence is traditionally patrilocal. This alignment between descent and post-marital residence means that the language which children are normatively expected to produce – the language of their patrilineal descent group – is also the language most widely spoken in the local community, easing acquisition of the target language. 

Indigenous migration to Catholic mission centers in the twentieth century and ongoing migration to urban areas along the Rio Negro in Brazil are reconfiguring the relationship between multilingualism and marriage. With out-migration from patrilineally-based villages, descent and post-marital residence are no longer aligned. Multilingualism is being rapidly eroded, with language shift from minority Eastern Tukanoan languages to Tukano being widespread. Continued practice of descent group exogamy even under such conditions of widespread language shift reflects how the semiotic relationship between language and descent group membership is conceptualized within the system of linguistic exogamy.

And, this 1983 book:

This book is primarily a study of the Bará or Fish People, one of several Tukanoan groups living in the Colombian Northwest Amazon. These people '...form part of an unusual network of intermarrying local communities scattered along the rivers of the region. Each community belongs to one of sixteen different groups that speak sixteen different languages, and marriages must take place between people not only from different communities but with different primary languages. In a network of this sort, which defies the usual label of 'tribe', social identity assumes a distinct and unusual configuration. In this book, Jean Jackson's incisive discussions of Bará marriage, kinship, spatial organization, and other features of the social and geographic landscape show how Tukanoans (as participants in the network are collectively known) conceptualize and tie together their universe of widely scattered communities, and how an individual's identity emerges in terms of relations with others' (back cover). Also discussed in the text are the effects of the Tukanoan's increasing dependency on the national and global political economy and their decreasing sense of self-worth and cultural autonomy.