Oster's research focuses generally on development economics and health. In 2005, Oster published a dissertation for her economics Ph.D. from Harvard University, which suggested that the unusually high ratio of men to women in China was partially due to the effects of the hepatitis B virus. "Hepatitis B and the Case of the Missing Women," pointed to findings that suggested areas with high Hep B rates tended to have higher male-to female birth ratios. Oster argued that the fact that Hep B can cause a woman to conceive male children more often than female, accounted for a bulk of the "missing women" in Amartya Sen's 1990 essay, "More Than 100 Million Women Are Missing." Oster noted that the use of Hep B vaccine in 1982 led to a sharp decline in the male-to-female birth ratio. Sen's essay had attributed the "missing women" to societal discrimination against girls and women in the form of the allocation of health, educational, and food resources. In April 2008, Oster released a working paper "Hepatitis B Does Not Explain Male-Biased Sex Ratios in China" in which she evaluated new data, which showed that her original research was incorrect. Freakonomics author Steven Levitt saw this as a sign of integrity.