Rajesh Kumar Singh/Associated Press

As India grapples with what seems like a constant barrage of shocking acts of violence against women, one question is asked again and again: Why is this happening?

One answer, some experts say, is India’s gender ratio, distorted by the practice of sex selection in favor of baby boys.

A much-cited 2002 study,“A Surplus of Men, a Deficit of Peace,” by Valerie M. Hudson and Andrea den Boer, contends that a gender imbalance in Asian countries, caused by a shortage of marriageable women, results in higher rates of crime, including rape, committed by young unmarried men.

“Internal instability is heightened in nations displaying exaggerated gender inequality, leading to an altered security calculus for the state,” the authors wrote in 2002, and reiterated in a book on the subject. Their conclusions are even more true today, Ms. Hudson said in an e-mail interview.

“Certainly the situation is, if anything, worse in both India and China than it was 10 years ago,” she wrote. “Certainly violent crime against women increases as the deficit of women increases. This will constrain the life chances of females far into the future.”

Right now, the statistics are worrying. India has 37 million more men than women, as of 2011 census data, and about 17 million excess men in the age group that commits most crimes, up from 7 million in 1991.

Violent crime in India rose nearly 19 percent from 2007 to 2011, while the kidnapping of women (much of which is related to forced marriage) increased 74 percent in that time. That’s a marked increase from the five years before 2007, when violent crime actually fell by 2.8 percent, and the kidnapping of women rose by 41 percent.

If the study’s conclusions are correct, India’s problems with rape and other forms of violence against women – recently seen in the gang rape and subsequent death of a 23-year-old woman on a bus in Delhi, the gang rape of a high school student in Bihar state and the rape of a young woman in Punjab, who committed suicide afterward – may only get worse, given the trend in India’s demographics.

The authors adopted a Chinese term, guang gun-er (“bare branches”), for unmarried men from age 15 to their mid-30s who have limited prospects for employment. This group, which is larger in countries where sex selection is prevalent, usually “commits the preponderance of violence within a society,” according to the report.

In a marriage market where women are scarce and thus able to “marry up,” certain characteristics of young surplus males are easily and accurately predicted. They are liable to come from the lowest socioeconomic class, be un- or underemployed, live a fairly nomadic or transient lifestyle with few ties to the communities in which they are working, and generally live and socialize with other bachelors. In sum, these young surplus males may be considered, relatively speaking, losers in societal competition.

Marital status affects more than just social standing for these men, the authors argued. Citing research indicating that levels of testosterone (referred to as “T” in the following quote) decline for married men, they said that marriage can thwart potentially antisocial male behavior.

When T falls, so does the propensity to engage in these behaviors. The more men in the society who are unable to marry, even though they would be willing to marry, the higher their circulating T and the greater amount of antisocial, violent and criminal behavior they will exhibit, generally speaking, than if they were able to marry.

A study on the sexuality of “bare branches” in China this year elaborated on the issues that arise with these carnally charged young men. The report said a “series of problems from sexual repression to sexual conflicts, from sexually transmitted diseases (including AIDS) to sexual crimes can arise.”

Intermingling and aggregation are key to understanding “bare branches,” according to Ms. Hudson and Ms. den Boer’s study. These men hang out together, befriend each other, compete with each other and legitimize each others’ “risky choices.” When clumped together and left to their own devices, they become a tool of social disorder, the authors said:

In this “least common denominator” theory, the behavior of men in groups — most particularly young, single, low-status males — will not rise above the behavior of the worst-behaved individual. Together, they will take larger risks and be more violent than they otherwise would individually. The sheer number of bare branches, coupled with the distinctive outcast subculture that binds them together and their lack of “stake” in the existing social order, predispose them to organized social banditry. The potential for intrasocietal violence is increased when society selects for bare branches, as certain Asian societies do. It is possible that this intrasocietal violence may have intersocietal consequences as well.

According to the 2011 crime statistics in India, of all the people arrested for rape crimes, almost 60 percent were men between the ages of 18 to 30 years and nearly 30 percent were men between the ages of 30 to 45 years.

India’s total sex ratio — defined as the number of females per 1,000 males — has increased over the past 20 years, after dropping for the 80 years before that. As of 2011, there were 940 Indian women for every 1,000 men, up from 933 in 2001. But, thanks to population growth and a still-prevalent practice of female foeticide, the number of “extra men” is growing among India’s youth. There will be about 30 million extra men in India between the age of 15 and 35, the study estimates.

And among India’s youngest population, the gender ratio is still getting worse, perhaps setting the foundation for new generations of violent crime and attacks on women, experts say.

India’s child population, defined in the national census data as all children between the ages of zero and 6 years, was almost 160 million as of 2011. The overall sex ratio for this age group is 914 female children for every 1,000 male children, and it is even more skewed in the urban parts, at 902. These figures mark a severe decline from a decade ago.

Overall, the Indian average gender ratio is far behind the global average of 984 for every 1,000 men, and is the second lowest in the world, before China. Urban India is on par with China though, with 926 women per 1,000 men.

India’s Planning Commission, in a report on women’s rights and child rights released last year, called the gender imbalance in the sex ratio “a silent demographic disaster in the making.”

The report said that the heavily patriarchal areas in the north and northwest have shown a mild improvement in the gender ratio for children, but that most of India has seen it decline. The number of female children relative to male children is expected to remain very low, according to the report.

The Indian government has tried to mend this deteriorating ratio through cash incentive programs that began in 2007. The idea, officials said, was to “force the families to look upon the girl as an asset rather than a liability since her very existence has led to cash inflow to the family.”

However, a recent evaluation of these various programs shows that they have way too many muddled conditions and imprecise focus groups.

“I think it is true that unless the government is willing to enforce its own laws against dowry and sex-selective abortion, not much will change,” Professor Hudson said.