A study this month reported a slight but steady decline in the ratio of boys to girls born in both the United States and Japan since 1970.
Normally, 105 boys are born for every 100 girls. Epidemiologist Devra Lee Davis and colleagues at the University of Pittsburgh, Pennsylvania, report that the decline is small, but the changes between 1970 and 2002 are equivalent to a shift from male to female of 125,000 babies in the U.S. and 135,000 in Japan.
Many industrial chemicals have estrogenic effects that can sabotage male gestation, the authors relate online in Environmental Health Perspectives. But increasing obesity, late-age childbearing, and the use of reproductive technologies could also have a hand. Nailing causes will require more detailed studies, they say. For example, researchers at the University of Ottawa, Canada, reported in 2005 that in the Aamjiwnaang First Nation community in Sarnia, Ontario, the sex ratio had declined to about 103 since the early 1990s--believed to be related to the tribe's proximity to petrochemical plants.
The decline in male births coincides with "other signs that male reproductive health is in danger," such as lower sperm counts, Davis warns. Harvard epidemiologist Marc Weisskopf says the study adds to evidence that "there are secular changes in sex ratio occurring," but the causes are still not clear.
