Forced bachelors, migration and HIV transmission risk in the context of China's gender imbalance: a meta-analysis.

China has experienced continual increase in the sex ratio at birth (SRB) since the 1980s, which has led to a serious gender imbalance. To identify whether the future forced bachelors, especially those who migrate to cities, will increase the risk of HIV spread, a systematic review was carried out of studies published since 2000 that include the sexual risks of male migrants of China. Five studies comparing risk differences between migrants and non-migrants showed male migrants had greater risk of having multiple sexual partners and engaging in commercial sex. Ten studies concerning the relationship between sexual risks and socio-demographic characteristics showed that unmarried male migrants were more likely to engage in commercial sex and be infected with STDs than married migrants, while male migrants with higher income were more likely to have multiple sexual partners and be infected with STDs. In an analysis stratified by sample characteristics, the association between marriage and sexual risk was greater among samples with lower mean age, higher average income and education. In addition, the risk selection on education and income disappeared in the samples of migrants of whom more than half were unmarried.