Marriage statistics
How marriage depends on social attitudes and economic incentives. (a) There may be two stable states that contain different proportions of married people for the same set of conditions (solid lines). In fact, these two branches are linked by a continuous curve (dotted line). However, beyond the turning points of the upper and lower solid curves, the states represented by the dotted curve are unstable. (b) This looped curve is exactly what emerges from van der Waals' theory of the liquid-gas phase transition. (c) A 3D graph shows the dependence of marriage on both social and economic factors. The loop in the curve appears only if the strength of social attitudes is strong enough. This plot is also familiar from van der Waals' theory, in which the inception of the kink in the surface marks the liquid-gas critical point (d). If "strength of social attitudes" is replaced by temperature, "economic incentives" by pressure, and "proportion of married population" by density, we have the phase space of a fluid.