by the fertility and mortality rates of a country. "Stationary" pyramid or constant population pyramid A pyramid can be described as stationary if the percentages of population (age and sex) remain approximately constant over time. In a stationary population, the numbers of births and death roughly balance one another. "Expansive" pyramid or Expanding population pyramid A population pyramid that is very wide at the younger ages, characteristic of countries with a high birth rate and perhaps low life expectancy. The population is said to be fast-growing, and the size of each birth cohort increases each year. "Constrictive" pyramid or Declining population A population pyramid that is narrowed at the bottom. The population is generally older on average, as the country has long life expectancy, a low death rate, but also a low birth rate. This may suggest that in future there may be a high dependency ratio due to reducing numbers at working ages. This is a typical pattern for a very developed country, with a high level of education, easy access to and incentive to use birth control, good health care, and few negative environmental factors. Youth bulge Gary Fuller (1995) described a youth bulge as a type of expansive pyramid. Gunnar Heinsohn (2003) argues that an excess in especially young adult male population predictably leads to social unrest, war, and terrorism, as the "third and fourth sons" that find no prestigious positions in their existing societies rationalize their impetus to compete by religion or political ideology. Heinsohn claims that most historical periods of social unrest lacking external triggers (such as rapid climatic changes or other catastrophic changes of the environment) and most genocides can be readily explained as a result of a built-up youth bulge. This factor has been also used to account for the Arab Spring events. Economic recessions, such as the Great Depression of the 1930s and the late 2000s Great Recession, are also claimed to be explained in part due to a large youth population who cannot find jobs. Youth bulge can be seen as one factor among many in explaining social unrest and uprisings in society. A 2016 study finds that youth bulges increase the chances of non-ethnic civil wars, but not ethnic civil wars. A large population of adolescents entering the labor force and electorate strains at the seams of the economy and polity, which were designed for smaller populations. This creates unemployment