This is why many demographers and sociologists believe the real trigger lies elsewhere, and it is pointed as squarely at China as it is at India: the growing proportion of single men.

Both countries are sitting on a demographic tinderbox caused by the profusion of what the Chinese call "bare branches" ( ), or men unable to find a spouse because of the paucity of women caused by an abnormally high sex ratio (males to females), which in turn is the product of the traditional preference for males in these societies. "Bare branches" are so called because they won't add to the family tree.

While crime - sexual or otherwise - is hardly the reserve of unmarried men, statistical evidence across the world shows a disproportionate amount of crimes are committed by this group. Societies that place a heavy premium on marriage and lineage only add to the frustration and low self-esteem of those who fail to start a family, raising the risk of violence within a group that is most prone to it.

India and China have similarly dire sex ratios at birth, creating millions of surplus males every year. In India, around 109 boys are born for every 100 girls, with the number going up to 120 in many parts. In China, the rate is around 120 boys, and up to 140 in provinces like Jiangxi and Henan .

The natural ratio is about 105 boys per 100 girls. Nature provides slightly more boys than girls at birth as males have higher mortality rates, so the ratio evens out by the time they reach reproductive age.

India and China have around 37 million excess men each. With the exception of Saudi Arabia, all other G20 countries have either more women than men or the numbers are almost equal.

"While rapes occur everywhere, predatory rapes such as the one in Delhi increase when sex ratios are abnormal. For years, researchers have noted that violent crime against women increases as the sex ratio becomes more masculine," says Texas A&M University Professor Valerie Hudson, who co-authored with Dr Andrea den Boer the seminal book .

Leading Chinese demographer Jiang Quanbao, from the Institute for Population and Development Studies at Xian Jiaotong University, agrees: "The Delhi rape can be linked to social, cultural, legal and judicial systems. But one neglected issue may be the distorted sex ratio."

Societies with abnormal sex ratios are considered inherently less secure. As Hudson and Den Boer find, the men who get to marry in such settings tend to have higher socioeconomic status. Those who can't are poorer, less educated and marginally employed. With little to lose, they exhibit a greater propensity for violence and more reckless behaviour, especially when they band together. Almost all the Delhi gang-rape suspects fit this description.

"These men are already at risk of establishing a system based on physical force in order to obtain by force what they cannot obtain legitimately. … Men who are not provided the opportunity to develop a vested interest in a system of law and order will gravitate towards a system based on physical force," Hudson and Den Boer wrote.

Marriage is a strong socialising force, says Jiang. "India should take note of the population's sex structure. The problem of bare branches is causing widespread concern for China's future domestic stability as well."

The lack of ready and reliable statistics on rape in China makes it hard to establish its exact link to excess men. But as sexual violence is more about violence than about sex, a rising pattern of any kind of violence rings alarm bells for demographers if it comes in lockstep with worsening sex ratios.

And according to Mara Hvistendahl, the Shanghai-based author of , it's not just violent crime. The skewed sex ratio should also be worrying because it has increased the trafficking of women in China.

Jiang found that the scale of abductions and trafficking of women was rising alarmingly. Fifty to 60 per cent of the total trafficking cases in China now involve the sex and entertainment industry. In the past, it was mostly for marriage.

Outward migration and trafficking from the hinterland to the richer, coastal provinces are only adding to the skewed sex ratios in the underdeveloped regions, leaving village after village teeming with depressed, restless men.

And it will get worse. Jiang's research shows that since 2010, the number of marriageable males in China has been increasing much more rapidly. Excess male population in the 20-49 age bracket will exceed 20 million in 2015, 30 million in 2025, 40 million in 2035 and 44 million around 2040. According to some estimates, between 2020 and 2050, 15 per cent of Chinese men will fail to find a spouse.

The problem of surplus men is umbilically linked to a phenomenon Nobel laureate Amartya Sen coined as "missing women" - a concept he developed in the early 1990s to calculate the number of extra women the developing world would have if these countries had the same sex ratio as in the developed world.

In absolute numbers, Sen found more than 100 million women were "missing" as a result of inequality and neglect that would lead to excess female mortality. China alone, he estimated then, had 50 million "missing women".

Several studies have since tried to quantify the number of "missing" women and have arrived at similarly stark numbers: 163 million girls in Asia in the past three decades, according to Hvistendahl; while some have calculated about two million each from India and China every year.

But there is a major difference. A much larger percentage - up to 45 per cent - of the missing women in China are lost before they are born. Prenatal factors account for around 11 per cent of the missing women in India, indicating the one-child policy has exacerbated traditional male preference in China by creating a strong incentive for feticide.

Sex-determination technology and easy access to abortion have made gender screening only easier. Following the introduction of the one-child policy in 1979, the 1982 census showed sex ratio at birth at 108.5 males per 100 females. This rose to 111.3 males in 1990, 116.9 in the 2000 census, and is now around 120.

Crime rates have risen in step. A study by researchers from the University of Chicago, Columbia University, the Chinese University of Hong Kong and Tsinghua University found that between 1988 and 2004, the number of surplus men in China aged 16-25 almost doubled. Crime rates nearly doubled as well. The rise in excess males accounted for one-seventh of the overall rise in criminality during the period.

But the "culling" of women, as Hudson puts it, has far wider ramifications than mere disruptions in law and order. Throughout China's history, a rising sex ratio has been a precursor to social and political unrest.

"The more important threat to the government would come if coalitional violence turned from the goal of taking resources, such as through theft and smuggling, to seizing local power, which could be a springboard for more ambitious aims," she says, citing the Nien rebellion in 1851 that shook the Qing dynasty.

The Nien rebels came from impoverished regions with a sex ratio of at least 129 men per 100 women, says Hudson. These small groups of bare branches started off with smuggling and extortion before forming armies to challenge imperial control.

In the 1920s and 1930s, polygamy coupled with infanticide created a high number of . Not surprisingly, this period also saw a surge of secret societies, bandit groups and cults. Jiang said many of the bandit groups recruited men with the promise of women captives.

More pressingly, rising crime can also act as a strong catalyst for social unrest by fanning mass discontent, which points to the vital role that sex ratio plays in keeping political order - especially in countries such as China with no institutional mechanism to channel anti-government rage. Even in a democracy as raucous as India, many saw shades of Egypt's Tahrir Square uprising in Delhi's impromptu street protests against the rape.

"A worsening sex ratio will intensify the instability of both India and China. And the threat to government rule may be greater for China than India," says Hudson, underlining the message a rape in distant Delhi carries for Beijing.