Much of this variation is because of underlying assumptions for "normal" birth sex ratios and expected post-birth mortality rates for men and women. Sen's original calculations using 1980s and 1990s data for missing women were indexed using the average sex ratio in Western Europe and North America as the natural sex ratio, through assuming that in these countries men and women received equal care. After further research, he updated these numbers with Sub-Saharan African sex ratios. Using these countries' sex ratios as the baseline and male-female populations from other countries as the data, he concluded that over 100 million women were missing, primarily in Asia. However, later authors pointed out that Europe tended to have higher rates of male mortality due to multiple wars and generally risky behavior. Due to male workers migrating from rural to urban regions, immigration, and world war, a culture of "high masculinity" existed in these countries, while on the other hand, in other countries such as India, traditions regarding the discriminatory treatment of female children were stronger from the late 1950s to mid-1980s. As a result of this disparity between countries, American demographer Coale re-estimated the Sen's original numbers of missing women using a different methodology. Using data from his Regional Model Life Tables, Coale found that the natural male-to-female sex ratio, accounting for different country fertility rates and circumstances, had an expected value of 1.059. Using the number, he then arrived at an estimate of 60 million missing women, much lower than Sen's original estimate. However, a few years later, Klasen re-calculated the count of missing women using Coale's methodology with updated data. He found 69.3 million missing women, which was higher than Coale's original estimate. He also noted a problem with the Regional Model Life Tables; they were based on countries with higher female mortality, which would bias Coale's numbers of missing women downwards. Furthermore, Klasen and Wink noted that both Sen's and Coale's methodologies were flawed because Sen and Coale assume that optimal sex ratios are constant across time and space, which they are often not. Klasen and Wink conducted a study in 2003 with updated census data. Using life expectancy to instrument for sex ratio at birth (which would account for non-constant sex ratios as well as biases from the Regional Model Life Tables), they estimated 101 million missing women across the world. Overall they found trends that showed that while