that the cumulative number of missing female births due to sex-selective abortion globally is 45 million from 1970 to 2017. Various researchers argue that declining fertility contributes to an intensified problem of missing women. This is because families have a preference for sons; a decrease in fertility would mean that families would no longer have children of multiple sexes, but rather a single male child. However, Klasen's research has found that other than in countries where policies severely restrict family planning (i.e. China due to the One Child Policy), fertility is not often associated with higher prevalence of missing women. This is because declining fertility is endogenous with other improvements of female well-being such as increasing female education, increasing female employment, and a decrease in gender bias. In fact, as Klasen notes, "In countries where fertility decline has been the largest, the share of missing women has fallen the most." However, this varies between countries. Das Gupta finds that in South Korea, the male-to-female sex ratio spiked from 1.07 to 1.15 between the 1980s and 1990s because of the rising prevalence of ultrasound technology for the use of sex-selective abortions, but declined afterwards between 1990 and 2000 because of increasing modernization, education, and economic opportunities. Furthermore, in a study contrasting India and Bangladesh, researchers found that India's declining fertility caused a large intensification in son preference and thus an increase in the number of missing women, while declining fertility in Bangladesh led to less missing women. Differential treatment and female bargaining power explanation Economist Nancy Qian shows that, in mainland China, the female deficit declines when women earn more, and argues that mothers' preferences for daughters and lower female bargaining power caused by lower wages can explain much of missing women in mainland China. Another well-known paper by economists Seema Jayachandran and Illyana Kuziemko that is published in the same journal, the Quarterly Journal of Economics, show that in India, mothers breastfeed sons longer than daughters, which contributes to missing women in India. Hepatitis B virus explanation In her PhD dissertation at Harvard, Emily Oster argued that Sen's hypothesis did not take account of the different rates of prevalence of the Hepatitis B virus between Asia and other parts of the world. Regions with higher rates of Hepatitis B infection tend to have higher ratios of male to female births for biological reasons which are not yet well understood, but