Recall from Chapter 8 the difference between relative prices and absolute prices, and how people make decisions based on the former, not the latter. Consider married couples with young children, who cannot be left alone, deciding on a night on the town, versus the same decision made by couples
without children. Suppose an expensive date including dinner and a concert is $150, whereas a cheap date is dinner and a movie costing $75. Each couple faces the same two options. The childless couple would have to sacrifice the enjoyment of two movies and two dinners for one expensive date
($150 / $75). The married couple, because they must hire a babysitter for, say, $50, no matter which date they decide on, will most likely choose the more expensive date. Why? Because the relative price is only 1.6 cheap dates ($150 + $50 / $75 + $50), compared to 2 for the childless couple. Therefore,
we would expect to see married couples on more expensive nights on the town. For as good as Alfred Marshall’s supply and demand scissors are, they are not much help in predicting this behavior. To illustrate, a BusinessWeek