Population dynamics are the patterns of change over time in populations. Populations fluctuate in response to fluctuating external forces, or because of the internal structure of the process of demographic renewal. Damped cycles one generation long may result from the interaction of random perturbation and the age distribution of reproduction. So-called Easterlin cycles two generations long, either damped or self-exciting, may arise from the lag between birth and labour force entry when fertility responds sensitively to labour market conditions. Longer-term dynamics arise from the interactions of population growth, capital, endogenous technology, and income.