Bill and Melinda Gates say the world could get an extra US economy's worth of income by making one important change for women

Pivotal Ventures

Melinda Gates in Dar Es Salam, Tanzania.

• Bill and Melinda Gates released their 2019 annual letter on Tuesday morning.
• The letter is a call to action; perhaps the biggest ask from the pair is a renewed commitment to boosting the world's "human capital," or people power.
• Melinda Gates points out that educating women would provide a serious economic stimulus.

Bill and Melinda Gates' 2019 annual letter, released Tuesday morning, is a call to action.

It suggests one simple economic stimulus plan that could net the world more than an entire US economy's worth of income.

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In a word, that solution is school.

It's not a new idea: Education is often touted as a great equalizer, a force that propels people to achieve their full potential.

But around the world, 30-year-old men have on average 10 years of education under their belts, while women have just nine. If women got a few more years of secondary schooling, the Gateses say, the world could achieve some serious financial gains.

"Girls' education, especially, is among the most powerful forces on the planet," Melinda said in the letter. "Educated girls are healthier."

They also earn a lot more cash.

"If all girls received 12 years of high-quality education, women's lifetime earnings would increase by as much as $30 trillion, which is bigger than the entire US economy," Melinda said.

Read More: Bill and Melinda Gates say the best way to put 'America first' is to invest in foreign aid - a not-so-subtle nudge at Trump

AP Photo/K.M. Chaudary

Pakistani girls living in Lahore's slums carry clean water ahead of International Women's Day in Pakistan, Wednesday, March 7, 2018.

Those earnings don't just boost those educated women individually. Closing the education gap between boys and girls boosts a country's GDP, and studies consistently find that women invest more of their income back into their families than men do. Whereas men contribute 35% of their earnings to family, on average, women returning 90% to their kin.

Educating women also has positive health repercussions that reverberate for generations.

According to UNESCO, "ensuring that girls stay in school is one of the most effective ways of averting child marriage and early births."

In other words, educated women are more likely to give birth to healthier babies who'll live longer. UNESCO estimates we could cut childhood deaths in half if all women were to receive a full 12 years of education.

"Educated mothers are more likely to ensure that their children receive the best nutrients to help them prevent or fight off ill health, know more about appropriate health and hygiene practices, and have more power in the home to make sure children's nutrition needs are met," a UNESCO fact sheet says.

Of course, providing a solid education to girls and women doesn't completely solve the gender disparity problem. Women with advanced degrees still have higher rates of unemployment than their male counterparts.

But as Gates Foundation CEO Sue Desmond-Hellmann recently told Business Insider, when women learn, their entire families, communities, and even countries benefit.

Because when a woman earns cash, "she'll spend money on health and education for her children," Desmond-Hellmann said.