It's often said that demographics are destiny. While the futures of nations are guided by much more than population trends, these demographic forces can play an awfully significant role. Countries need to grow in order to stay healthy and successful, but not too quickly or they risk problems like political instability. Bigger populations can mean bigger economies (and bigger militaries), but only if the state can provide the necessary infrastructure and services. More people means more pressure on natural resources, but it can also mean more businesses, more exports, more tax revenue.

All of that makes the above map really important for how we think about the trajectories of nations and of the world. It shows current trends in population growth, based on data from a new United Nations Population Fund report. The blue countries are growing, with darker blue countries growing very rapidly. Purple countries are growing slowly or are about stagnant, with less than 1 percent population growth every year. Red countries are actually shrinking, typically because people are leaving and/or because they're not having enough babies, and that can cause huge problems.

A note on the data: The researchers estimate current population growth by averaging the annual change in each country's population, per year, between 2010 and 2015. Those latter few years are obviously projections, but with the rare exception for a major war or natural disaster, such short-term projections are considered reliable. The numbers include births and deaths, as well as migration -- an important point we'll touch on later. Here are a few noteworthy trends from the data.

(1) The big story is sub-Saharan Africa

Almost all of the countries growing more than 2 percent per year are in Africa. There are a few reasons for that. Birth rates there tend to be very high. People are living much longer as health standards improve. And the continent is becoming more stable and more peaceful, meaning that there are fewer wars, famines and natural disasters. All of these trends are happening at once and are poised to completely transform Africa. This chart, from a different U.N. agency's population projections, show that Africa's growth is expected to outpace the rest of the world's for the next century, more than quadrupling in size:

Africa's amazing growth could bring great opportunity for the countries there, as well as significant risk. Natural resources will be stretched, risking returns to instability, and as more people move into cities, the demand for social services will go up. But those same trends could see standards of living rise significantly. More immediately, as the U.N. report points out, an unusually high number of sub-Saharan births are among girls age 15 to 19. That is, quite simply, a bad thing: Young mothers are at greater health risk, as are their children, and they are less likely to receive an education or enter the work.

(2) Continued rapid growth in the Middle East

The Arab Middle East has been experiencing a very significant youth bulge over the last few years, meaning that an unusually large share of the population is young people. That's been problematic because when large numbers of young people hit working age at the same time, the Middle Eastern economies aren't able to provide enough jobs. Combine that with the region's stale authoritarian regimes and it's a recipe for political turmoil. That's considered a major factor in the unrest since 2011. With populations there still growing quickly, those trends could continue for some time, if Arab governments are able to get it together.

The highest growth rates in the Middle East -- indeed, in the world -- are actually in the Middle Eastern countries that have seen the most stability during the Arab Spring. The absolute highest growth rate, by far, is in Oman, with 7.9 percent growth annually. That's just staggeringly high; there's no telling what it could mean for the country. Super-rich Qatar has a growth rate of 5.9 percent, also extremely high, though it's likely the tiny Gulf state can use its tremendous oil and gas wealth to absorb the impact. But keep an eye on Jordan: Its 3.5 percent annual growth rate, combined with its poverty and its problematic influx of Syrian refugees, could bring real instability to the little kingdom that has resisted democratization for so long.

(3) Eastern Europe is a demographic disaster (and so is Japan)

When your population is shrinking, that's bad for several reasons. First, it means your population is not growing, which you really need it to be in order to sustain a healthily growing economy. Second, it means that working-age people will make up an ever-smaller share of your population, which will be increasingly dominated by the elderly. Old people typically don't work, and they consume far more social services. So the strain on those services goes up just as the number of people paying into them goes down. It's a recipe for real trouble.

Most of the shrinking countries are in Eastern Europe, where birth rates are very low and lots of people are leaving to seek better opportunities. Some post-Soviet European countries are shrinking up to 0.8 percent per year. That's a lot. The only other shrinking countries are Germany, Japan and Cuba, all at just 0.1 percent per year. But Japan's population decline is expected to accelerate dramatically, as this chart shows:

Japan is the world's third-largest economy. Its shrinking and increasingly elderly population is expected to cause serious trouble for that economy, which also happens to be currently burdened with huge amounts of public debt. That has some economists worried that the country may face a demographic crisis that could become an economic crisis as well. Ironically, many people would like to migrate to Japan and Germany, which would seriously ameliorate their demographic woes, but both countries have strict policies limiting immigration.

(4) Immigration is counteracting the West's demographic slowdown

If you look at Europe and North America, you'll see that birth rates there are pretty low, typically quite close to zero. That's bad news; while too-fast population growth is bad, countries do want some growth, and after centuries of booms, the Western world is seeing that growth slow. It's expected to reverse in a number of countries; Germany is perhaps just the leading trend of looming Western demographic decline.

The exceptions to this trend are countries that have lots of immigration: Ireland, Iceland, Norway are all above 1 percent average annual growth. The United Kingdom and United States, though marked purple, are in the upper end of that range, with 0.6 percent and 0.8 percent annual growth, respectively. That's just a difference of a single percentage point per year, or less, but over time it can make a big difference. Check out this chart showing the projected populations of Western European countries plus South Korea, a similarly developed country:

The trend lines for countries with little immigration are down, meaning eventually they will turn red on a map like the one at the top of this page. The trend lines for countries that foster more immigration tip upward, a much healthier trajectory.

(5) Growth in East Asia is slowing

After a world-changing demographic explosion, East Asia is seeing birth rates slow. It wasn't long ago that countries such as China, South Korea and Burma would all be deep, dark blue on this map. In fact, public health has gone way up in these countries, with people living longer and dying less frequently, which should drive home that birth rates are really coming down in East Asia.

That is, to some extent, healthy. As China's leaders knew when they enacted the deeply controversial "one-child policy," limiting all parents to a single child, the country's population had been growing faster than the economy and government could sustain. The danger comes if the population slowdown continues, as East Asians become more affluent (richer people tend to have fewer babies), and countries like China and South Korea risk a Japan-style demographic crisis. But that is at least a generation away.