Describe Robert Trivers Theory of Parental Investment (Trivers 1972) and critically evaluate the way in which it has been used to understand human psychological processes. The theory if correct has far reaching application in explaining the psychology and behaviour of the human animal when it comes to the driving force of reproduction and indeed all of human society which if evolutionary theory is correct is formed around the driving force to reproduce and pass on ones genetic material. In humans it is the females which provide greater parental investment both by virtue of the size of their gametes and as fertilization is internal leading to a 9 month gestation period followed by up to 4 years of lactation, in the energy and resources required for these processes. Human males have very little initial investment, however since selection forces have occurred to promote male parental care, human males pursue strategies that involve investing in their offspring to ensure maximum survival of that offspring as well as a tendency to mate with other fertile females that he does not intend to invest in, maximising his chances of reproductive success. Trivers refers to this as a mixed strategy. The selection process for male paternal care would have involved both the selection pressures by the female in preferentially mating with males showing signs of processing traits that would make them good fathers and a male desire to ensure that their offspring were viable and able to pass on their genetic material. These processes at all times would have remained implicit and although would have shaped cognitive processes would not have consciously done so. A large area in which it has application in understanding human psychological processes is in the sex differences observed within human mate preferences. A major study carried out by David Buss (1989) in this area showed a tendency for females from many different cultures to rate male resources as highly desirable in a...