While these may not be sufficient to affect reproductive success, the fact that the costs and benefits are not merely hypothetical can be used to test the prediction that altruistic behaviour is mediated by Hamilton's rule. We do not seek to contrast preferences for kin with other explanations for altruism, such as prosociality or reciprocity, whether direct (e.g.
If participants follow Hamilton's rule, investment (time for which the position was held) should increase with the recipient's relatedness to the participant.
There was a main effect of recipient (F(3, 63) = 8.34, MSE = 0.06, p < .01, [[eta].sub.P.sup.2] = .28, all p values relating to tests of the directionality Hamilton's rule's predictions about kinship effects are one-tailed, all others are two-tailed).
Second, evolutionary theory does not consider relatedness as the only criterion for investment under Hamilton's rule. Since the central issue is maximizing the number of copies of a gene propagated into future generations, the age of the beneficiary is a critical consideration: older individuals inevitably produce fewer future offspring than younger ones and therefore have a lower reproductive value (Fisher, 1930), making them of potentially less interest for altruistic investment (for reviews, see Barrett, Dunbar, & Lycett, 2002; Hughes, 1988).
In other words, Hamilton's rule is not limited to the peculiar cultural environment now characteristic of the post-industrialized West.
As such, they provide the first compelling experimental evidence that humans abide by Hamilton's rule when making judgments about how to behave towards others.
The first type of Hamilton's rule arises in social groups in which participants have correlated phenotypes.
The second type of Hamilton's rule arises when the fitness consequences of a phenotype can be divided into distinct components.
The two types of Hamilton's rule have coefficients described by statistical regressions.
I show that the direct fitness form of Hamilton's rule has the same logical status as FTNS: it is an exact, partial condition for change ascribed to social selection.
If we use the Fisherian definition of partial change caused directly by natural selection, holding average effects constant, then the right side is zero and we recover the standard form of Hamilton's rule. This form of Hamilton's rule is an exact, partial result that applies to all selective systems, just as the partial frequency fundamental theorem is an exact, partial result with universal scope.
This form of Hamilton's rule is a purely phenotypic result.