The environment: pollutants/toxins, the area one lives in.
Ethnicity: Chinese women had the highest ratio of boys to girls, followed by Filipino women, while American Indian moms had the lowest.
Others include the age of the parents, stress, and lower maternal weight (Centers for Disease Control).
Michigan State University said that human beings can be affected sexually by the chemicals, which cause sex-related defects in animals and resulting in more boys. PCBs are endocrine disruptor's (chemicals that act like hormones). They and related chemicals known as dioxins can cause deformity of the sexual organs in fish and alligators, and cancer, infertility and perhaps other sexual changes in humans.
First link found between latitude and sex of babies:
A global survey of birth data shows that more girls are born in the tropics than anywhere else, suggesting for the first time that latitude can affect the gender of fetuses. Reasons could be in the variations in temperature and the day length, but there is no definitive answer. The results fit in the countries where parents selectively abort girls. Of the 20 countries with the lowest ratios of boys to girls, 18 were at tropical latitudes. And even though the explanation in China is selective abortion of girls, it has been shown that the latitude correlation held up when all Asian and African countries were excluded.
The Weaker Sex:
According to The New York Times in “The Weaker Sex”, although more boys are born, it appears they may be more susceptible to a host of diseases, injuries and more, and soon women outnumber men.
Facts on Females Living Longer & Stronger:
Boys have a higher death rate if born premature: 22 percent compared with 15 percent for girls.
More newborn males die than females (5 to 4).
Sudden infant death syndrome is one and a half times more common in boys than girls.
As teenagers, boys die at twice the rate of girls.
Men have fewer infection-fighting T-cells and are thought to have weaker immune systems than women.
By the age of 36, women outnumber men.
Stroke, cancer, diabetes, heart disease and accidents--all among the top causes of death--kill men at a higher rate than women.
American men typically die almost six years before women do.
By the age of 100, women outnumber men eight to one.
During fetal development “the male sex is clearly the more fragile one,” said Bruce B. Allan, an obstetrician-gynecologist in Alberta Canada.
Gender selective abortion:
The long-held preference for boys decreases the number of successful female births. Families from all stages of economic life oftentimes abort their girls, because although advanced education can decrease the preference for boys, it can also increase the couple's knowledge of tests early on. Also, economic reasons can drive this decisions, which is discussed later on.