Empirical Testing of Evolutionary Hypotheses

In order to understand how evolutionary psychologists test hypotheses, it is necessary to consider the hierarchy of levels of evolutionary analysis depicted in Figure 8.1. At the top of the hierarchy is evolution by selection. The theory has been tested directly in many cases. New species can be formed in the laboratory by its application, and dogs can be selectively bred using its principles. Since there has never been a single case in which the general theory has been proved to be incorrect, most scientists take the general theory for granted and proceed with a more specific form o hypothesis testing.

At the next level down are middle-level evolutionary theories, such as the theory of parental investment and sexual selection. According to this theory, the sex (male or female) that invests more in of fspring is predicted to be more discriminating or "choosy" about its mating partners. And the sex (male or female) that invests less in offspring is predicted to be more competitive with members of its own sex for sexual access to the high-investing sex. From these hypotheses, a number of specific pre dictions can be derived and tested empirically. In the human case, for example, women bear the heavy parental investment burdens of internal fertilization and nine-month pregnancy. Women are the high-investing sex; thus, according to the theory , they should exert more selectivity in their choice of mates than should men, who require

H1: higher investing sex is more selective

Theory of reciprocal altruism

Evolution by selection

H2: females select based on ability and willingness to invest

Theory of parental investment and sexual selection

H2: females select based on ability and willingness to invest

H3: lower investing sex competitive for access

Theory of parasite-host co-evolution

H3: lower investing sex competitive for access

Figure 8.1

Evolutionary analysis hierarchy, depicting the conceptual levels of evolutionary analysis. At the top of the hierarchy is natural selection theory. At the next level down are middle-level evolutionary theories from which specific hypotheses and predictions can be derived. Each level of the hierarchy is evaluated by th cumulative weight of the empirical evidence from tests of the predictions derived from it. Source: Adapted from Buss, 1995a.

only the contribution of sperm in order to reproduce. Two specific predictions can b derived from this hypothesis: (1) women will choose as mates men who are willing to invest resources in them and their children and (2) women will divorce men who fail to continue providing resources to them and their children.

Using this method of deriving specific testable predictions, researchers can can-out the normal scientific business of empirical research. If the data fail to support th predictions and hypotheses, then the middle-level theory from which they were derived is called into question. If the findings, when tested many times by indepen dent researchers, support the predictions and hypotheses, then the middle-level theory from which they were derived increases in credibility .

The deductive reasoning approach, or the "top down," theory-driven method of empirical research is one approach to scientific investigation. Another method, which is equally valid, is called the inductive reasoning approach, or the "bottom-up," data-driven method of empirical research. In the inductive reasoning approach, a phenomenon is first observed, and then the researchers look for or develop a the ory to fit the observations. Just as astronomers observed the galaxies in the univers expanding before they had a theory to explain why , psychologists notice and empirically document a number of phenomena before they have theories to explain them. In the domain of personality , for example, we might notice that men tend to be more physically aggressive than women. Although nothing in the theory of evolution by selection would have predicted this sex dif ference in advance, it is fair game for subsequent theorizing. The dual inductive and deductive approaches, of course, can apply to all theories in personality psychology , not just evolutionary theories.

Once a theory is proposed to explain the sex dif ference in aggression, however, we can ask, "If the theory is true, then what further predictions follow from it that we have not already observed?" It is in these further deduced predictions that the value and tenability of the theory rest. If the theory generates a wealth of deductive predictions, which are then confirmed empiricall , we know that we are on the right explanatory track. If the theory fails to generate further testable predictions, or if its predictions fail to be confirmed empiricall , then the theory is called into question. For example, one theory of sexual aggression against women has proposed that men who have experienced deprivation of sexual access to women are more likely to use aggressive tactics. This has been called the mate deprivation theory (Lalumiere et al., 1996). The evidence, however, has failed to support this hypothesis—men who have difficulty attracting women are no more likely to use sexual aggression than are me who are highly successful at attracting women. The mate deprivation theory , in short, appears to be false.

Evolutionary hypotheses have sometimes been criticized as being vague, speculative "just-so stories," implying that they are like fairy tales that have little scientific value. There is some justification for this criticism, and, in the early days o evolutionary psychology, there were more armchair speculators than empirical scientists. Recently, however, evolutionary hypotheses have been framed in a precise and testable manner, so this criticism is no longer valid (Buss, 2004; Buss, 2005; Kenrick & Luce, 2004). All the standards of normal science hold in evaluating evolutionary psychological hypotheses. Individual scientists bear a responsibility to formulate the evolutionary hypotheses in as precise and testable manner as possible.

With this theoretical background in mind, let' s now turn to the implications of an evolutionary perspective for the three key levels of personality analysis—human nature, sex dif ferences, and individual dif ferences.

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