Bill and Melinda Gates revealed their 9 biggest surprises from 2018 in a letter dedicated to Microsoft's late cofounder

Ted S. Warren/AP

Bill and Melinda Gates.

• Bill and Melinda Gates published their annual letter for 2019.
• The Gates' listed their nine biggest surprises of in 2018, which include home DNA-tests catching out serial killers, sexist data, and the fact that toilets remain largely unchanged.
• They dedicated the letter to Bill Gates' Microsoft cofounder Paul Allen, who died in October 2018.

Bill and Melinda Gates have released their 2019 annual letter, and its theme is "surprises."

"How would you describe 2018? Was it what you expected? We'd probably say no," the letter begins. "From especially devastating natural disasters on the one hand to record numbers of women campaigning for office on the other, 2018 felt to us like a series of surprises."

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The Gates' go on to say that while some events have been welcome, others have been less pleasant. Here are the nine things that surprised Bill and Melinda this year:

1 . Africa is the youngest continent, with a median age of 18

Bill points out that everywhere else in the world the average age is going up. "This can be either an asset or a source of instability. Melinda and I believe that the right investments will unlock the continent's enormous potential. Young Africans will shape the future of not only their own communities but the entire world," writes Bill.

2. At-home DNA tests can find serial killers - and could also help prevent premature birth

Bill referred in particular to the 2018 capture of the Golden State Killer, 33 years after his crimes. The suspected killer was arrested by investigators after a distant relative of his uploaded their DNA to an open-source DNA-sharing website called GEDmatch.

Read more: The suspected Golden State Killer was finally caught because his relative's DNA was available on a genealogy website

Gates also writes that a large sample provided by 23andMe users gave scientists a clue as to which genes can cause premature labour.

3. We will build an entire New York City every month until 2060.

Gates warns against climate change being exacerbated by electricity usage, manufacturing, and agriculture.

Read more: Bill Gates warns of the dangers of cow farts - and the world should take his words seriously

"It's not realistic to think that people will simply stop using fertilizer, running cargo ships, building offices, or flying airplanes. Nor is it fair to ask developing countries to curtail their growth for the sake of everyone else," writes Gates.

"Part of the solution is to invest in innovation in all five sectors so we can do these things without destroying the climate. We need breakthrough inventions in each of the grand challenges."

4. Data can be sexist

The Gates' expressed concern about the reams of missing data about women, particularly in developing countries. Not only is missing data a worry, but Melinda writes that the way data is collected about women can stack the odds against them.

Read more: Why it's totally unsurprising that Amazon's recruitment AI was biased against women

"We like to think of data as being objective, but the answers we get are often shaped by the questions we ask. When those questions are biased, the data is too," she writes.

5. You can learn a lot about processing your anger from teenage boys

The Gates' talked in-depth about attending a meeting of high-school boys talking about how they deal with their anger. Bill said he learned a lot from watching those boys - some of whom had lived through tragedy - find ways of handling their anger.

Read more: After talking to teens about anger management, Bill Gates admits that some of his Microsoft meltdowns were over the top

6. There's a nationalist case for globalism

In a more political section, Bill and Melinda argued that nationalism doesn't necessarily exclude international cooperation.

"There is nothing about putting your country first that requires turning your back on the rest of the world. If anything, the opposite is true," Melinda writes.

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

Bill specifically appealed for international cooperation when it comes to the treatment of diseases.

7. Toilets haven't changed in a century

The Gates' described a "toilet fair" which they organised in Beijing last year - looking for a new design to oust the flush toilet.

"Several companies are business-ready. Their inventions check almost all the boxes: They kill pathogens, can keep pace with the needs of fast-growing urban areas, and don't require sewer infrastructure, external water sources, or continuous electricity to operate," writes Bill.

He added the only problem at the moment is affordability, to which end the Gates Foundation will be investing in more R&D.

8. Textbooks are becoming obsolete

Bill said that the thing killing off the textbook is very same invention which helped make his fortune: Software.

"When I told you about this type of software in previous letters, it was mostly speculative. But now I can report that these tools have been adopted in thousands of U.S. classrooms from kindergarten through high school. Zearn, i-Ready, and LearnZillion are examples of digital curricula used by students and teachers throughout the US," he writes.

9. Mobile phones are most powerful in the hands of the poorest women

"For the world's most marginalized women, a mobile phone doesn't just make their old life more convenient; it can help them build an entirely new life," writes Melinda. However, she pointed to a gender gap in phone ownership in poorer countries.

Finally: The letter is dedicated to Bill's Microsoft cofounder

As a signoff, Bill and Melinda dedicated this year's letter to Paul Allen, who died in October 2018 at the age of 65. They said:

"Paul was a brilliant man with a wide-ranging mind and a special talent for explaining complicated subjects in a simple way. He loved to share his passion for music, science, the arts, sports, philanthropy, and so much more.

"He supported homeless shelters, brain research, and arts education. He helped us see how much good innovation could do in the world. He deserved more time in life, and his passing left a big hole in our hearts. We'll think of him every time we hear Jimi Hendrix."

Paul Allen's love for Jimi Hendrix was well documented, and he even built a museum devoted to the rock legend.

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