There is strong evidence that humans also display population cycles. Societies as diverse as those of England and France during the Roman, medieval and early modern eras, of Egypt during Greco-Roman and Ottoman rule, and of various dynasties in China all showed similar patterns of political instability and violence becoming considerably more common after times of relative peace, prosperity, and sustained population growth. Quantitatively, periods of unrest included many times more events of instability per decade and occurred when the population was declining rather than increasing. Pre-industrial agrarian societies typically faced instability after one or two centuries of stability. However, a population approaching its carrying capacity alone is not enough to trigger general decline if the people remained united and the ruling class strong. Other factors had to be involved, such as having more aspirants for positions of the elite than the society could realistically support (elite overproduction), which led to social strife, and chronic inflation, which caused incomes to fall and threatened the fiscal health of the state. In particular, an excess in especially young adult male population predictably led to social unrest and violence, as the third and higher-order parity sons had trouble realizing their economic desires and became more open to extreme ideas and actions. Adults in their 20s are especially prone to radicalization. Most historical periods of social unrest lacking external triggers, such as natural calamities, and most genocides can be readily explained as a result of a built-up youth bulge. As these trends intensified, they jeopardized the social fabric, thereby facilitating the decline.