People, and the statistical study of them. In the 200 years since Thomas Malthus forecast that population growth would result in mass starvation, dire predictions based on demographic trends have come to be taken with a pinch of salt. Even so, demography does matter. In developed countries, economists have studied the impact of the post-war “baby-boomer” population bulge as it has grown older. In the 1980s, as the bulge dominated the workforce, it may have contributed to a sharp, if temporary, rise in unemployment in many countries. Boomers starting to save for retirement may have increased demand for shares, so fuelling the bull stock market of the 1990s; as they retire and sell their shares for spending money, they may cause a long bear market. Furthermore, as they become elderly and retire, health-care spending and retirement pensions are likely to eat up a growing share of GDP. To the extent that these are provided by the state, this will mean increasing public spending and higher taxes. But whether they are provided by the state or by the private sector, the ageing of baby-boomers will impose a growing financial burden on the younger workers that have to support them (see replacement rate). Economists have tried to measure the extent of this burden using generational accounting, which looks at the amount of wealth transferred from one generation to another over the lifetimes of the members of each generation. Economists have also developed many different theories to explain why populations grow and why the fertility rate slowed sharply, to below the replacement rate, in many developed countries during the 1990s. One explanation is based on the notion that people have children so that there is somebody to look after them in old age. Fertility rates fell because the state increasingly looked after retired people, and infant mortality rates were lower so fewer births were required to ensure that there were some children around in the parental dotage. Also, with a lower probability of a child dying, it paid the parents to have fewer children and to channel their energy and resources into maximizing the human capital of the few. Alternatively, it may have had something to do with an important innovation: the cheap and easy availability of reliable contraception.