Parental investment is any contribution a parent makes to a child to increase his or her chances of survival and reproduction at the expense of the parent's ability to contribute to other aspects of his or her suitability (Barrett et al., 2002). The investment may be time, energy, food, protection, shelter, or other forms of care for the benefit of the offspring (Barrett et al., 2002). Costs to the parent are incurred on its inclusive fitness through decreasing its ability to reproduce or invest in other existing offspring (Barrett et al., 2002). Human parental investment is unique in two respects, both best understood from an evolutionary perspective. First, the amount of parental investment is high compared to other primates. Second, fathers share a greater proportion of parental investment in humans than in most other mammals. The large amount of parental investment in humans arises from the high energetic cost of growing and maintaining brain tissue, combined with the large size of the human brain and its relative immaturity. at birth. One unit of brain tissue requires over 22 times more metabolic energy than an equivalent unit of muscle tissue (Aiello, 1997). Furthermore, the human brain is six times larger than would be expected for a primate of its same body size, and primates already have large brains compared to other similarly sized mammals (Barrett et al., 2002). Furthermore, the human brain is one-quarter the size at birth, compared to that of great apes, which is half the size at birth (Barrett et al., 2002). Therefore, the human brain after birth starts out relatively underdeveloped and has a greater amount of growth to accomplish, a process that has high metabolic costs. This report is the basis... half of the document... 1997). Brains and guts in human evolution: The expensive tissue hypothesis. Brazilian Journal of Genetics, 20, 141-148Barret, L., Dunbar, R., & Lycett, J. (2002). Human evolutionary psychology. Princeton, New Jersey: Princeton University PressByrne, R.W., & Whiten, A. (1988). Machiavellian intelligence: Social competence and the evolution of intellect in apes, monkeys, and humans. Oxford: Oxford University PressCunnane, S. C., & Crawford, M. A. (2003). Survival of the fattest: Fat babies were key to the evolution of the large human brain. Comparative Biochemistry and Physiology, 136, 17-26Geary, D.C. (2005). Evolution of paternal investment. In D. M. Buss (Ed.), The handbook of evolutionary psychology (pp. 483-505). Hoboken, New Jersey: John Wiley & Sons Marlowe, F. (2000). Paternal investment and the human mating system. Behavioral processes, 51, 45-61