A world without women? In her book Unnatural Selection: Choosing Boys Over Girls, and the Consequences of a World Full of Men, journalist Mara Hvistendahl argues that it's certainly a possibility. According to her research, the world is experiencing a "demographic shift that is tilting our population in favor of men." In China, 120 boys are born for every 100 girls. The ratio is 108 to 100 in India, and 110 to 100 in South Korea. What accounts for this "sex imbalance," and what are its ramifications? Here, a brief guide:

Why is this happening?

Families in many cultures have long preferred sons to daughters, says Laura Blue at TIME. Sons often earn the family more money, while women may be married off to other families, taking with them expensive dowries, says Maureen Callahan at the New York Post. Cheap, readily available ultrasounds can reveal the sex of a fetus early on in a pregnancy, leading to a rise in abortions in countries like India and China. Parents who want a boy will abort a female after seeing the results of the ultrasound, says Blue, "so that they can try again for a male without increasing the size of their family."

How common is this?

There are laws against sex-selection abortions, but still, a study in the medical journal The Lancet reports that 600,000 female fetuses are aborted in India each year.

What are the ramifications for a dwindling female population?

"It's not like [when] salt becomes scarce and the price goes up," says Hvistendahl, as quoted by TIME. "It doesn't work that way with women." Instead, practices like bride buying and prostitution will increase, she predicts. Already, in places with the most skewed ratios, "honor crimes" like the killing of sisters who have "dishonored their family" have increased, says Joshua Kurlantzick at The National.

And what would happen to men?

A world with a surplus of men could be more violent, says Blue, especially since testosterone is often linked to "risk-taking behavior and aggression." Past populations in which men greatly outnumbered women are remembered for being "rough-and-tumble" and unruly. "Think of the Wild West," Blue says. A sex-ratio map of the U.S. in 1870 closely resembles that of China today. Marriage "makes men more peaceable," says Kurlantzick, but within a decade, 30 million men in China may not be able to find wives.

Can this trend be stopped?

China and India are doing their best to encourage the birth of baby girls, says Callahan, by toughening their laws and instituting fines. In Fuji, parents who have two girls get a $150 annual pension for life, as well as health care, housing, and education benefits. India announced in 2008 that parents of girls who complete school and stay unmarried until after age 18 would be given $5,000 cash.

Sources: Salon, TIME, New York Post, National