A critique of the grandmother hypotheses: Old and new

The singularity of reproductive senescence in human females has led many in vestigators to consider menopause an adaptation permitting increased matern al investment in existing progeny. Much of the focus has been on the grandm other hypothesis-the notion that aging women gain an inclusive fitness adva ntage from investing in their grandchildren. This hypothesis has evolved fr om an explanation for menopause into an explanation for the exceptionally l ong postreproductive lifespan in human females. In the old grandmother hypo thesis, menopause is an adaptation facilitating grandmothering; it is about stopping early in order to create a postreproductive lifespan. In the new grandmother hypothesis, grandmothering is an adaptation facilitating increa sed longevity, and menopause is a byproduct. This paper reviews and critica lly evaluates the evidence for and against both hypotheses, focusing on key predictions of each. If menopause is the result of selection for increased maternal investment, this involved mainly mothers, not grandmothers. (C) 2 001 Wiley-Liss, Inc.