A family cycle past a government's advertisement board in support of baby girls in Morena, India. Early results show India has only 914 girls under age 6 for every 1,000 boys. (Mustafa Quraishi/AP)

The practice of couples in India aborting female fetuses if they don’t yet have a male child is becoming more common and widespread, according to a new study.

Sex-selective abortion is intended to increase a couple’s chances of having at least one boy. The growth of the practice appears to be an unintended product of India’s rising educational attainment and prosperity reaching all corners of the country, the study suggests.

The phenomenon of “missing girls” is especially evident among second-born children in families in which the older child is a girl. There doesn’t appear to be a strong preference for boys as the first-born child, as there is among some population groups in China where one-child families are the norm.

“It appears that families are saying, ‘Nature will decide the first child, but we are going to let technology decide the second child if the first is a girl,’ ” said Prabhat Jha, an epidemiologist and demographer at the University of Toronto. Jha led the research, which was published online Tuesday by The Lancet.

He said the “unusually specific” nature of sex selection in India represents not so much a devaluation of girls’ worth as a strong desire to have at least one boy, particularly at a time when most couples are choosing to have fewer children than did their forebears. It is also the consequence of advanced medical care — in particular of sonograms during pregnancy — becoming available to millions more women.

Jha said he expects the findings to be controversial in India, where “selective abortion of girls is commonly known but seldom discussed.”

Since 1996, India has banned the use of medical technology to determine the sex of fetuses in order to selectively abort girls. But the findings suggest it is now happening more often than in the past, said Subu V. Subramanian of the Harvard School of Public Health, who wrote a commentary accompanying the study.

“There is a whole role of the medical community that has been overlooked,” he said. “The penalties for breaking the law are certainly not sufficient enough to deter this.”

Jha and his collaborators at four government and research institutes in India used data from the last three decennial censuses and from epidemiological surveys in which women were asked the birth order and sex of their children.

Humans bear slightly fewer girls than boys — about 950 to 975 girls for every 1,000 boys. Boys are somewhat more likely to die in the first months of life than are girls. By school age in industrialized countries, there are roughly equal numbers of each sex.

The study found that, from 1990 to 2005, the “sex ratio” of first-born female children in India did not change significantly nor differ from what was biologically expected. (In 1990, it was 943 girls per 1,000 boys, and in 2005 it was 966). However, in families whose first-born was a girl, the incidence of the second-born being a girl fell almost steadily over that period, from 906 per 1,000 boys in 1990 to 836 in 2005.

During the period, the trend increased among families in which the mother had 10 or more years of education but did not change in families in which the mother had no education. The sex ratio fell especially sharply in the richest 20 percent of households, Jha and his colleagues found. The findings were the same in both Hindu and Muslim households.

The most extreme decline in the probability of having a girl occurred in families in which the first two children were girls. In that case, the ratio of girls to boys in the third-born child was 768 to 1,000 in 2006. This came at a time when the average family size in India was 2.6 children — a huge reduction from earlier generations.

The overall phenomenon of many more boys than girls among children under age 6 was once limited to northern and western India. Now it has spread throughout the country, Jha said. In 1991, about 10 percent of India’s population lived in states where the sex ratio for girls was below 915. Today, 56 percent of the population does.

In parts of Punjab, Gujarat and Haryana states — in the north and northwest of India — “the absolute gap is as great as has been observed in any other part of the world, if not greater,” Jha said.

While the phenomenon of parental preference for boys is common throughout the world, it is most obvious in Asia — particularly in China, Korea and India. However, it exists subtly in the United States.

In an article in the journal Review of Economic Studies in 2008, two University of California researchers, Gordon B. Dahl and Enrico Moretti, observed several phenomena that appear to evince favoritism for boy offspring.

In the United States, parents of first-born girls are more likely to get divorced than parents of first-born boys, suggesting the latter have greater investment in family stability. After divorce, fathers are more likely to obtain custody of sons than of daughters.