While there is a clear correlation between poverty and fertility rates at both country and household level, it is more difficult to distinguish the causality at work between the two variables: does poverty cause families to have more children, or, conversely, is high fertility responsible for perpetuating poverty and economic underdevelopment? Alternatively, are poverty and fertility levels the effects of different causal chains? Scientists are not of the same mind when providing answers to these questions. In the academic literature, there have been relatively few attempts to take into consideration demographic factors when carrying out economic analyses of the causes of poverty.
However, correlations are easy to see, as illustrated in Figure 6 below. The vast majority of high-income societies have average fertility rates below or at the replacement level of 2.1 children per woman, with few exceptions. In higher-middle-income countries, fertility rates cluster between one and three children, with some exceptions above three. Several of the outliers in these two groups are oil-rich countries. On the other hand, lower-middle-income countries have most widely distributed fertility rates, while those at low-income levels are the big majority of cases, at over four children per woman.