Traditional norms of human societies in rural China may have changed due to population expansion, rapid development of the tourism economy and globalization since the 1990s; people from different ethnic groups might adopt cultural traits from outside their group or lose their own culture at different rates. Human behavioural ecology can help to explain adoption of outgroup cultural values. We compared the adoption of four cultural values, specifically speaking outgroup languages/mother tongue and wearing jeans, in two co-residing ethnic groups, the Mosuo and Han. Both groups are learning outgroup traits, including each other’s languages through contact in economic activities, education and social networks, but only the Mosuo are starting to lose their own language. Males are more likely to adopt outgroup values than females in both groups. Females of the two groups are no different in speaking Mandarin and wearing jeans, whereas males do differ, with Mosuo males being keener to adopt them than Han males. The reason might be that Mosuo men experience more reproductive competition over mates than others, as Mosuo men have larger reproductive skews than others. Moreover, Mosuo men but not others gain fitness benefits from the adoption of Mandarin (they start reproducing earlier than non-speakers).

This article is part of the theme issue 'Social norm change: from evolution to policy intervention'.

Cultural characteristics of 1,720 Mosuo and 712 Han people, who presented during the investigation, were collected in a single-round survey, along with a demographic and socio-economic survey of this population in 2017. The cultural survey included questions on speaking languages and dressing style. Here we focused on four cultural characteristics varying across group and spatial scales (Table 1), Mandarin (0=cannot speak, 1=can speak), Sichuan dialect (0=cannot speak, 1=can speak), Naru (0=cannot speak, 1=can speak), and wearing of jeans (1 =never wear, 2 = rarely wear, 3 = often wear, 4 = always wear).

We used demographic data to calculate reproductive success and to identify biological kin and affinal kin networks. We calculated the number of offspring living to age 15 for elder Mosuo and Han men and women born between 1911 and 1950. We also computed the number of alive children (of any age) and age at first birth for all the adult participants aged 15 or more in the cultural survey. We also defined the intermarriage status of all the participants (0 = no, 1= intermarried with Han, 2= intermarried with other ethnic groups, and only those ever married were included).

Only part of the data used in the paper is presented due to the need to preserve anonymity.