The current consensus is that we'll reach our maximum population by around 2050 and then start to slowly decline, but that might be based on two critically flawed assumptions. The United Nations estimates that the human population will reach nine billion people right around the year 2050. Considering how fast we went from six to seven billion people—and it also took only twelve years to get from five to six billion—it might seem strange that it could take nearly forty years to add another two billion. It's because the birth rate is actually negative in several post-industrial, generally affluent countries in southern and eastern Europe, and this trend is expected to spread outwards.