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donors, while offspring are % related. Hamilton thus predicts grandoffspring to receive investment only when benefit/cost ratio is double. My own analysis allows more role for group selection, without saying how much, and shifts attention from who benefits to when. Petty’s idea, if | understand him, is years purchase = generation length = 21years, (7.5) which would give 1 generation length 21 years cash flow rate = 4.7% /year. (7.6) This would tally well enough with rates of return and interest rates as Petty knew them. I would adjust Petty’s estimate of the generational length. Petty’s primogeniture model may have been true to law and custom for land inheritance, but it is not true to biology. I prefer R. A. Fisher’s? method equal-weighting all births from first to last, and equal-weighing ages of both parents at each birth. We have some evidence that the maternal generation length in recent decades, by that method, has run near 26 years over recent decades. If fathers are five years older on average, Fisher’s method would arrive at 28.5 years. Rogers found 28.9 years from other sources. Then (7.6) would give cash flow rate = i. 3.5% /year. (7.7) 28.5 years 3 The Genetical Theory of Natural Selection (1930). Chapter 7 Petty’s Idea 2/3/16 19 HOUSE_OVERSIGHT_011078