"It is now causing widespread social disruption as entire societies are masculinised," writes Joni Seager.
Ms Seager details how in 1995 just six countries had gender imbalances. It's now more than 20, with South Korea the only country to have reversed its distorted ratio in the last 30 years.
"Among other consequences, a shortage of women seem to be contributing to local and regional increases in trafficking and kidnapping of women," she said.
"Son preference reflects the combined forces of economics, culture and religion.
"As smaller families become the norm, the pressure to have sons accelerates.
"Girls are widely considered to have a lower economic value than boys — a view that is often strengthened by marriage, dowry and inheritance practices."
Ms Seager said son preference used to be thought of as a practice for the poor but increasing affluence suggested the opposite.
Her survey of the latest global data shows in the US 49 per cent of men would prefer a son, over 22 per cent for a girl and 28 per cent not bothered. For women there isn't too much of a difference.
In India the rate of girls per boys up to six years old has decreased, with 91.8 girls per 100 boys in 2011 compared to 94.5 in 1991.
In China the problem is worse. It's become so skewed that it's now about 118 boys per 100 girls compared to 109 in 1982.
The natural rate is 105 boys per 100 girls.
Other countries where the figure is high include Vietnam with 112 and Azerbaijan with 116.
The missing women are split between Nigeria, Pakistan, India, Bangladesh, Indonesia and China.
Nigeria is the lowest at two million and China the highest at 68 million.
The atlas suggests infanticide could be responsible in countries including Hong Kong, Vanuatu, Taiwan, Macedonia and Egypt because there rates of boys per 100 girls is well above the natural rate for zero to four year olds.
