producers' and consumers' durable goods, yielding a stream of benefits which are compared with a stream of costs. Births occur if the present value of the benefits exceeds the present value of the costs. The theory has been used by Malthusians to rationalize the casual observation that population growth rates increase during the early stages of economic development and then, perhaps, fall later on as

Dear Old Friend: As you have doubtless observed, it has been many a week now since I last stepped out of the shadows, put the barrel to your ribs and divested you of cash, trousers, and credit cards. You have probably asked yourself, Where is my faithful old stickup man, John? Doesn't he like me any more?-The truth is, sir, that I'm terrified you'll take it amiss and move out of New York if I overwork my welcome. John [Quoted in Russell Baker's column, New York Times, December 9, 1975]