Overall, there are slightly more men than women in the world. According to 2015 estimates by the United Nations, there are 101.8 men for every 100 women, with the number of men rising gradually each year since 1960.

But a map by the Pew Research Center with the latest U.N. data clearly shows that men and women are distributed unequally around the globe. In former Soviet republics, for example, women outnumber men. Conversely, there are more men than women in Asia, Arab countries and Northern Africa.

Latvia, Lithuania, Armenia, Belarus, Russia, Ukraine and Estonia are among the countries with the largest female populations. The same countries also lead world statistics in the life-expectancy gap between men and women. In Belarus, men have a life expectancy of 65.3 years, while it's 77 years for women. Only war-torn Syria has a bigger life-expectancy gap between men and women.

The national differences between male and female populations change with age. In Russia, for example, there are more newborn boys than girls each year, and men continue to outnumber women until age 31. But from age 32 onward, there are more women than men, with the gap widening every year.

Much of the gender discrepancy can be explained by history. The demographics of the former Soviet republics have been greatly influenced by their 2oth-century history. According to the very first census conducted in Russia, there were 98.9 men for every 100 women in 1897. It almost matches today's gender ratio in the United States. (98.3 men for 100 women).

[Map: Where Europe is growing and where it is shrinking]

The proportion of women in Russia started to rise during World War I, and the growth continued through the Russian civil war, famine and the "Great Terror" in the Soviet Union. According to the 1939 census, there were 91.9 men for every 100 women in Russia. Then World War II had a devastating effect on the population of the Soviet Union, with a disproportionate number of men falling in the conflict. In 1959, there were 81.9 men for every 100 women in the Soviet Union, with a big gap between parts of the state directly affected by the war (79.7 men for every 100 women in Ukraine) and those not affected by it (92.3 in Uzbekistan). The gender ratio has since improved. In 1989, shortly before the collapse of the Soviet Union, there were 89.5 men per 100 women there. It was 88.1 in the Russian territories.

The gap started to grow again in the 1990s because of early mortality among men. Most men who die young in Russia do so because of drinking and alcohol-related incidents, Health Minister Veronika Skvortsova said last year.

The imbalance between the sexes is in reverse in China and India, countries known for sex-selective abortions and female infanticide. There are 106.3 men for every 100 women in China and 107.6 in India. This kind of discrepancy can lead to problems such as higher violence and homicide rates, scientists argue. Chinese authorities, concerned by this, have toughened punishment for sex-selective abortion and provided extra pensions for parents of girls in rural areas.

The gap is even wider in the United Arab Emirates and Qatar, with 274 and 265.5 men, respectively, for every 100 women. The main reason for this is that a large number of foreign workers in the Persian Gulf states leave their families back in their native land.

Read more:

5 ways the world will look dramatically different in 2100

India will have the world's largest population earlier than expected, U.N. says

Map: If the size of countries reflected their populations