| Chapter 2: | Background on Population Sex Ratio |
|
|
However, these results were not replicated in a nationally representative sample of births from India that included a large number of both vegetarian and nonvegetarian women (Arnold & Roy, 2001). Similarly, parental age gap has been hypothesized as a causal factor in altered sex ratio at birth (Manning, Anderton, & Shutt, 1997), but no association was found between the difference in parents’ ages and sex ratio at birth when very large national samples were analyzed (Arnold et al., 1997) or in a large sample from northern Italy (Astolfi & Zonta, 1999).
There is some evidence that acute psychological stress may result in a lower sex ratio: In Slovenia 6 to 9 months after the 1991 war, a significant decrease in the sex ratio at birth was detected (Zorn, Sucur, Stare, & Meden-Vrtovec, 2002). The investigators analyzed the motility of sperm over time among men with baseline normal sperm levels from fertility clinic records and found a decline in sperm mobility during the time of the war. A decline in the sex ratio at birth was found after the earthquake in Kobe, Japan (Fukuda, Fukuda, Shimizu, & Moller, 1998). A Kurdish city in Iran had a decline in the sex ratio at birth following a mustard gas attack (Saadat, 2006). James’ hypothesis, that parental hormone levels around the time of conception affect the sex of the fetus, provide a mechanism for these study findings (James, 2004).
Recent studies have examined how perinatal viral infections may differentially affect male and female fetuses. In a study of the vertical transmission of human immunodeficiency virus (HIV) in Malawi, Taha et al. (2005) found that girls were more likely to be born infected (12.6%) than boys (6.3%).