Half a million girls a year are being aborted in India, equal to the total number of girls born in the UK. The scale of sex selection in Asia is extraordinary, yet it has not attracted the attention it deserves in the west. This is the age of "missing women" - an estimated 30 to 70 million of them. The number of girls born has dropped significantly because of sex selective abortions, and the sex imbalance has been aggravated by higher mortality rates as boys receive better care.

The very vagueness of the figures indicates how poorly understood this massive phenomenon is – quite apart from the still limited analysis of the impact the resulting sex imbalance will have on the development of countries such as India and China. In May, there was widespread concern in India as the results of the 2011 census revealed a sharp deterioration of the sex ratio. Many attributed the growing gap to a powerful lobby of doctors and commercial interests who sell ultrasound machines; both make profits from the illegal, multimillion dollar sex selection procedures.

An ultrasound scan costs the equivalent of £10, a small amount relative to the cost of a dowry expected with a girl. Even for less well-off families, a sex-selective abortion may seem cost effective, according to a study by Sonia Bhalotra, from the Centre for Market and Public Organisation (CMPO) at Bristol University, published in the summer issue of its Research in Public Policy bulletin.

But what is particularly interesting about her analysis is that the practice is more strongly established in the better off areas of India than the poorer rural areas. This challenges assumptions that the practice is driven by traditional customs. In fact, it is the wealthy and better educated who are most likely to choose sons, perhaps because they want to keep their families small. One of the richest states in India is Punjab, where, in the 2001 census, there were 125 boys per 100 girls in the nought to six age group. The predominant pattern is to use sex selection not for the first child but for subsequent children; the ideal seems to be two sons and one daughter.

There is also evidence that high-caste Hindus are more likely to use the practice than Muslims. Bhalotra suggests that cultural practices - parents live with their sons until death and it is the son who lights the funeral pyre - may be among the factors driving sex selection.

The economic boom in India is accelerating the trend of sex-selective abortion. This factor is combining with technological developments in ultrasound that make scans more reliable earlier in pregnancy, and the machinery more mobile, increasing penetration across the country. The research talks of an "intersection of recent economic, demographic and technological change with deep rooted social preference", which is costing the lives of millions of girls.

There are no easy steps to stop the practice. Sex-selective abortion has been illegal since 1994, but has little impact on the growing trend. Proposals to ban ultrasound machines are unlikely to gather substantial support because the devices also play a positive role in pre-natal care.

Another study from the CMPO looked at differential forms of care for girls and boys in India. Using the Indian Time Use Survey, they found that, on average, boys receive more parental time a day than girls – an extra 30 minutes , or 14%. The quality of childcare is also better for boys, who are more likely to be vaccinated and given vitamin supplements, and are typically breastfed for longer. These inputs were on average 10% higher for boys. The research concludes that such differential care probably accounts for nearly a third of the higher mortality rate for girls.

The research doesn't offer detailed analysis of the consequences of this gender imbalance, other than to suggest that a large number of unmarried men is likely to lead to an increase in violence against women. It will be another 15 years before we begin to see the political and social impact of the missing women.