strategies and how they relate to life history trade-offs. Trivers' parental investment is defined as investment in offspring to that benefits their survival and ability to reproduce, at the expense of the parent's ability to invest in other offspring. Inherent in these strategies is an underlying trade-off between energy and investment allocation to oneself as a parent and to each offspring. Paternal investment is more variable than maternal investment worldwide, and compared to other primates paternal investment is more robust in humans. Mating and pair-bonding includes trade-offs such as making a choice between investing in current offspring, or investing in future mating opportunities. Over the course of human evolution, there is evidence of reduced sexual dimorphism in humans compared to other primates. This suggests that there was less male-male competition for female mates, which led to more male investment in offspring, rather than mate choices. Paternal investment strategies vary facultatively based on alloparental care, the costs and benefits of offspring investment, societal pressures, divisions of labor, cultural expectations and norms, and the individual qualities of males in any given society. In the field of reproductive ecology, it has been a recent interest to explore the endocrinology of social relationships, including the relation of paternal investment and endocrine function. It has been shown that fatherhood in general, reduces testosterone levels and competition for mates increases testosterone. It is also shown that male endocrine function is mediated by interactions with children. Maternal investment is widespread and less variable than paternal investment, but there have been recent evidence supporting multiple mating systems for females as well in the evolutionary literature. This could suggest that mating systems may influence how maternal investment is given, and the trade-offs posed both biologically and socially. Maternal investment is almost always necessary for the survival of offspring, because compared to other primates, human infants are highly altricial. Offspring are also categorized as taking longer to wean, still dependent after weaning, and a longer juvenile period. Parent offspring conflict is a theory synthesized by Trivers in the 1970s alongside parental investment. Parental offspring conflict is also well documented and develops in tandem with the process of reproduction and parenting. Parent-offspring conflict occurs in the relationship between parent and fetus (in the case of striking a balance between allocating placental energy stores to the growing fetus, while maintaining and metabolic balance of the mothers biology), and between parent and offspring.