2.1. Study area

Data collection was carried out over two field sessions in the Sichuan Province of China in 2018 and 2019. Consent was sought from each individual interviewed and from local People's Government at each site, and the research was approved by Lanzhou University Life Sciences and UCL Research Ethics committee. In each site, different ethnic groups were interviewed, with Mosuo, Han and Yi women being interviewed around Lugu Lake and Zhaba women in Daofu County. These different ethnic groups have likely been living this way for thousands of years (Dong, Yu, & Liu, 2008) and therefore serve as good models for testing the intragenomic conflict hypothesis. The Mosuo and Zhaba women display an uncommon duolocal residence pattern with matrilineal descent, where neither sex leaves the natal group for marriage, and descent is traced through the female line. As an alternative to cohabiting marriage, they engage in the practice of zǒu hūn (“walking marriage,” 走婚), in which males and females live in their natal households that comprise of many generations of family members (He, Wi, Ji, Tao, & Mace, 2016; Ji et al., 2013; Wu, Ji, et al., 2015), and men only visit their wives and girlfriends during the night (Cai, 2001). Men have little or no financial obligations to their spouses or children, and they tend to invest heavily in their sisters' offspring (He, Wu, Ji, Tao, & Mace, 2016; Ji et al., 2016). A family planning policy established in the 1980s meant that marriage became more important for reproduction, and therefore, partnerships in the Mosuo and Zhaba are slightly more formal now than they were in previous years; however, in most cases, the husband and wife still live apart (Ji et al., 2016; Thomas et al., 2018). As a contrast to these groups with little female dispersal, the Han and the Yi display patrilocality accompanied by patriliny where descent is traced down the male line, and women leave their natal home to join that of their husbands' kin at marriage.

As residence patterns were always somewhat flexible (Ly et al., 2018) and have become more fluid in recent years, both due to policy changes and increased tourism (Mattison, 2010), this enables us to consider the individuals ancestral residence pattern (based on their ethnic group) in addition to the way in which they are currently living. The intragenomic conflict hypothesis predicts that ancestral residence pattern should be influential over menopause timing and symptoms (Úbeda et al., 2014), however, if current residence pattern better predicts variation in menopause timing and symptoms, then it would suggest something else is responsible for the diversity we see in the menopausal transition.