Two recent events - the horrendous gang rape in Delhi in December and the all-India bandh organised by trade unions - provide the two bookends for the theme of this column: the dangerous demographic time bomb ticking away, not only in India but also in China.

This relates to what the Chinese call "bare branches": males who cannot find spouses because of high sex ratios (males/100 females). To delineate this demographic time bomb the sex ratio for those under 15 is most relevant, since it provides the relevant population of young males in the next two decades. According to the Central Intelligence Agency's World Factbook, this ratio was 117 for China, and 113 for India.

What are the likely consequences of these distorted sex ratios in India and China? An important book by Valerie Hudson and Andrea den Boer, Bare Branches: The Security Implications of Asia's Surplus Male Population (MIT, 2005), provides a historical survey and some disquieting answers. The most skewed sex ratios are in northern India and parts of China (which have been exacerbated by the country's one-child policy). In the northern Indian states of Punjab and Haryana, the sex ratios for 0-6 years population in 2011 were 118 and 120, respectively; in Delhi, it was 115. This should be compared to the all-India ratio of 109. Historically, when the sex ratio of the 15-34 age group is close to 120, and if there is resource scarcity and a lack of productive jobs, there is likely to be "chronic violence and persistent social disorder and corruption". By 2020, the authors estimate, there will be 28 million to 32 million "surplus" males in the 15-34 age group in India, and between 29 million and 33 million in China.

How will these countries cope with these huge numbers of often low-status, testosterone-fuelled, and most often economically challenged males in the prime of their youth? The history of societies with such gender imbalances is not encouraging for the maintenance of internal and external order.

Since anthropologist Napoleon Chagnon's 1968 study of a Stone Age Brazilian tribe (Yananomo: The Fierce People), it has been known that much primitive warfare was not (as on the standard anthropological view) about material possessions; as a shaman he was interrogating on the subject who grew exasperated at his traditional anthropologist's questions expostulated: "Don't ask such stupid questions! Women! Women! Women! Women! Women!" The sex ratio for the below-15 age group among the Yananomo was 128.6. The resulting bare branches are augmented by the polygamous monopolisation of the available women by successful high-status warriors.

In 19th-century Rajasthan and Oudh, female infanticide was common. For Rajput clans, kidnapping was often the only way to get wives for their sons (page 214). While in Oudh, where the juvenile sex ratio in 1875 was reported to be 118.6, W H Sleeman, the scourge of the Thugees (immortalised in John Masters' novel The Deceivers), reported how bands of bare branches rebelled openly against the government. One particularly bloody mutiny by these robber-barons led to the British annexation of Oudh.

In the 19th century, China was also plagued by armed revolts by bands of bare branches. The most serious was the Nien rebellion in 1851, when an organised group of bandits from the poor northeastern province of Huai-pei combined with the Taiping rebels in the south to form an army of over 100,000 bare branches, which nearly overthrew the Qing dynasty. The sex ratio in 19th-century Huai-pei has been estimated to have been 129 (page 208).

Bare branches share many characteristics in high- sex ratio societies. They are in the lowest socio-economic class; they are more likely to be underemployed or unemployed; they are usually landless with few other resources that would improve their chances of marrying; they are largely transients with few ties to communities where they seek work; they live and socialise with other bare branches, creating distinctive bachelor subcultures. Often treated as social outcasts, they are prone to vice and violence. This is linked to the higher testosterone levels of unmarried men compared with married men (page 196).

Unemployment and lack of good jobs exacerbate these tendencies (page 197). It is in this context that the failure to reform the colonial-era labour laws, with their restrictions on exit and the concomitant monopolisation of good formal sector jobs by "insiders" creating a virtual aristocracy of labour, has prevented India - unlike China - from making full use of its most abundant resource for labour-intensive industrialisation, as was devastatingly outlined by Raghuram Rajan in the recent Economic Survey. The increased informalisation of Indian labour markets will worsen the dangers from India's bare branches. Labour market reform remains imperative, not only to raise India's growth rate, but also to tame the impending social disorder from its bare branches.

We can also expect to see the other social and political consequences found in high-sex ratio societies in India: increased prostitution, homosexuality, the growth of monastic orders and polyandry, as well as the capture and trafficking of women. In India, because of the large regional differences in sex ratios, marriage migration of women from the South and the Northeast is already happening and is likely to increase. This will alleviate the problem of bare branches in India - unlike China, where there is resistance to taking alien brides, thanks to its relatively homogeneous Han culture. The reported trafficking of girls from Southeast Asia is more likely to be for prostitution.

But it is the political consequences that are most disturbing. The authors conclude that historically there have been two ways in which governments have sought to quell the disorder flowing from a large number of bare branches. The first is to organise them to be exported abroad through colonisation or war, while suppressing disorder at home through authoritarian means. This is most likely to be China's route, with its massive increase in spending on defence and internal security. The second, in culturally diverse societies, is to direct bare branch violence towards various minorities. This, the authors conjecture, is the most likely outcome for India. But there is a third possibility, with both China and India mobilising their bare branches in an arms race, whose denouement of mutual male slaughter culls their "surplus" males. Welcome to the Dangerous New World of emergent Asia.