Magazine in 1988 called "Girls from Space," found that both high altitude tactical pilots and astronauts fathered far more daughters than sons. A 2006 letter published by BMJ, however, reviewing some of the research, claims that "So far, researchers have found no morphological differences between human X sperm and Y sperm", ignoring prior findings to the contrary, including some published in the same journal just the year before. A May 2021 article in Fertility & Sterility reflected on a paper published in the same journal 50 years earlier that month, stating that "...scientific journals such as Fertility and Sterility and The New England Journal of Medicine have continued to publish research dispelling the claims of the Shettles Method." References Fertility