One of the most underappreciated forces affecting life in the U.S. is the sex ratio. Generally defined as the number of males per 100 females, it has a profound bearing on several issues, not least of which is the status of women.
Before World War I, immigrants, who tended to be predominantly male, kept the ratio high. Restrictive legislation in the 1920s and the Great Depression of the 1930s reduced the influx to a trickle. Beginning in the 1940s, lung cancer and cardiovascular disease, both of which affected men far more than women, resulted in an increasing proportion of females. The rise in the ratio since 1970 has resulted from a greater reduction in mortality among males than females. Increasing illegal immigration, which brings in more males than females, has apparently offset the effect of legal immigration, which now tends to involve more females than males.