1. The majority of America is no longer middle class
For the first time in four decades, the share of Americans living in middle-income households fell below 51 percent in 2015, according to an analysis of government data by the Pew Research Center.
In early 2015, the United States had 121.3 million adults in both lower- and upper-income households combined, compared with only 120.8 million in the middle-class, Pew says.
Inequality is growing on both ends of the wealth spectrum. The turning point occurred because American households are both falling and rising out of the middle class, as the chart below shows.
2. Gun deaths became as common as traffic deaths in the U.S.
Americans are now as likely to die because of guns as cars, according to new government data issued at the end of 2015. Figures from the Centers for Disease Control and Prevention showed that guns and traffic accidents each killed about 34,000 people last year in the United States, or about 93 people per day.
The rate at which people die in motor vehicles in the United States has been falling for decades, due to better safety features in cars, tighter regulations on drunk driving and seat beat use, and more effective emergency health care.
The story for guns is more complicated. Gun deaths fell sharply in the 1990s but have recently plateaued, due to a combination of falling gun homicide and rising gun suicide rates.
3. Global poverty fell to its lowest point ever
Stories of war, crime and terrorism often make it seem like the world is going to hell in a handbasket, but there are uplifting stories out there. Here's one: Based on World Bank projections, the proportion of people around the world living in absolute poverty fell to its lowest point in history in 2015.
As the chart below from Max Roser of Our World in Data shows, only 9.6 percent of the world's population was living in absolute poverty this year, which the World Bank defines as living on less than $1.90 per day in 2011 prices. That might sound like a lot still, but it's a stunning drop from fewer than 50 years ago, when roughly 60 percent of the global population lived in absolute poverty. .
4. More Mexican immigrants are now leaving the U.S. than arriving
This year marked a turning point in immigration that caught many Americans by surprise: the tide in Mexican migration shifted. Data from Pew Research Center showed that, since 2009, 140,000 more Mexican migrants had left the United States than arrived.
As the chart below, by The Post's Philip Bump shows, the trend is a sharp divergence from 1995-2000, when 2.27 million more Mexican migrants entered the United States than left it.
5. The U.S. saw the most deaths by Islamic terrorism since 2001
The massacre in San Bernardino, Calif. — where 14 people were killed and 21 were wounded — was so deadly that it altered the picture of terrorism in America. According to data from the New America Foundation, a Washington think tank, 2015 was the deadliest year for Islamic terrorism since the Sept. 11, 2001 attacks. Nineteen people died in the United States at the hands of Islamic terrorists in 2015, including the San Bernardino shooting and another shooting in Chattanooga, Tenn., in July.
As the chart below shows, however, the death toll in the United States from right-wing terrorists -- a category that includes white supremacists, anti-government fanatics and the Colorado Planned Parenthood shooting deaths -- still remains higher than that for Islamic terrorists since Sept. 11, 2001.
6. A million migrants entered Europe
The number of refugees and migrants who poured into Europe this year has topped 1 million, according to newly released estimates from the International Organization for Migration. The vast majority arrived in Europe via the Mediterranean Sea, as the chart below shows. More than half of the migrants came from Syria, while 14 percent were from Afghanistan.
7. Africa went a year without naturally occurring polio
As Bill Gates pointed out in a recent blog, 2015 was also a turning point for public health, with Africa marking one full year without a single new case of "wild polio" -- polio that is acquired locally and naturally. That victory was the result of a massive and remarkable public health campaign that began in 1988, when polio was endemic in 288 countries, Gates says.
It's a fragile victory: Polio is still endemic to Afghanistan and Pakistan, and two cases appeared in Europe this year for the first time since 2010. But as Gates argues, the world has gone "99 percent of the way to eradication."
8. China scrapped its one-child policy
China announced in October that it would finally end the 36-year-old regulation that limits many couples to having only one child. While most families are now limited to two children, the change still marks a significant increase in personal freedom, and a much-needed step toward increasing the country's working-age population.
Yet the regulation will continue to leave deep scars, including demographic imbalances and memories of forced sterilization and abortion. As the chart below shows, China's population has already aged significantly, and lifting the one-child policy probably won't do too much to change that.
9. The Fed raised interest rates, marking the end of an era
The Federal Reserve's rate hike in December was notable not for its size -- the federal funds rate increased by a tiny one quarter of a percentage point -- but for its symbolism. It was the first rate hike in more than a decade and a sign that some of America's most prominent economic thinkers believe the economy is once again sound, roughly seven years after the onset of the global financial crisis.
10. Millennials surpassed boomers as the biggest U.S. generation
This was the year that baby boomers finally lost their position as the country's largest generation, a title they had held for decades. According to tracking from Pew, the United States was projected to have 75.3 million millennials (defined as those between the ages of 18 and 34) in 2015, compared to 74.9 million boomers (51-69) and roughly 66 million Gen-Xers (35-50).
11. Same-sex marriage became a right in the U.S.
A historic Supreme Court decision in June ruled that state bans on marriage among same-sex couples are unconstitutional. The 5-to-4 decision capped off a rapid change in public opinion about gay marriage over the past decade. Before the decision, 13 states had banned gay marriage. As the map below shows, that is no longer the case.