According to evolutionary psychologists, the primary, or even sole, determinant of our behavior is the result of certain behavioral characteristics being selectively advantageous relative to a given environment.
In other words, the reason we possess certain behavioral characteristics is because they were useful for our ancestors, and those who did not possess such characteristics were naturally selected out of the environment because their behavioral characteristics did not allow them to efficiently adapt to the environment.
Interestingly enough, while evolution is accepted by secular philosophers, many philosophers of biology and philosophers of science are highly critical of many elements of evolutionary psychology. Downes (2014) cites Cosmides and Tooby as summarizing the most central tenets of evolutionary psychology in the following ways:
1) Brains are computers that are designed by natural selection, and their purpose is to select (useful) information from the environment.
2) The behavior of humans is the product of this evolved computer, and this evolution is the result of certain kinds of responses to the information it extracts. We come to understand this behavior by understanding cognitive programs which result in this behavior.
3) These programs are adaptive. The reason they have endured is because this programming has allowed them to endure and reproduce.
4) While at least certain elements of these programs may not be adaptive in our current environment, the reason they currently exist is because they were adaptive for our ancestors and were thus passed down to us.
5) The brain consists of numerous distinct programs resulting from this process of natural selection. This is called the "massive modularity thesis." It says that the human brain consists of these distinct programs rather than one overarching program. Just as specific organs serve specific functions rather than there being general purpose organs, so also, there exist in the program naturally selected specific-purpose programs rather than general-purpose programs. According to such theorists, since organs with specific purposes are selected for in response to specific kinds of environmental stimuli, this also follows for different parts of the brain. The notion of the brain as a fully integrated, indivisible module would be as absurd, evolutionary psychologists argue, as the idea of a general purpose organ. From a less biological perspective, distinct programs are specific responses to specific programs, and therefore evolve in a distinct manner and for distinct purposes.
6) Being able to understand explain the computational architecture of the evolved human brain allows us to formulate a comprehensive and systematic understanding of individual and social phenomena. This principle is oftentimes criticized as being overly reductionistic.
Hypotheses in evolutionary psychology are tested in a manner similar to the testing of ordinary psychological hypotheses. For example, an evolutionary psychologist may believe that male preferences for women with specific waist-to-hip ratios is the result of men seeking our fertile females. Thesey psychologists interview male subjects from various cultures and countries and argue that unity in preference among these varied populations demonstrates that such universality indicates that these preferences are selectively advantageous rather than an accident of history or culture. Of course, many psychologists point out that universality itself is no proof that this universality is the result of a specific selective tendency.
As noted before, there are many critics of evolutionary psychology from within the discipline of philosophy of biology. Many philosophers of biology are particularly critical of evolutionary psychology's supposed over-reliance on various degrees and forms of adaptationism. For example, evolutionary psychologists tend to uncritically assume that all traits, behavioral or not, must have been the result of natural selection. This is more an assumption than something which is straightforwardly empirically verifiable. Critics of evolutionary psychologists point to numerous vestigial organs or traits which organisms possess, which survive no adaptive value.
Critics of evolutionary psychology also point to the important distinction between ontogenic and phylogenetic adaptations. While evolutionary psychologists emphasize the importance of phylogenetic adaptations, they run the risk of ignoring or downplaying ontogenetic adaptations, which are learned during one's lifetime rather than resulting from 'innate' biological traits acquired through selective inheritance through one's ancestors.
Indeed, evolutionary psychology proceeds by way of a kind of reverse engineering. Evolutionary psychologists make inferences about the adaptive occasion of the inheritance and acquisition of certain traits based on their function. Since such and such a trait, in other words, serves such and such a function, it must have evolved relative to an environment in which such a function was adaptively useful.
Of course, the danger here is that evolutionary psychology threatens to compromise its status as a science and devolve into mere ideology, in a manner similar to how Marx and Freud believed that their systems were scientific, yet they simply imposed their hermeneutic scheme on all phenomena and accommodated it to the framework rather than attempting to impartially examine the evidence for the validity of the interpretive scheme itself.
Rather than constituting an objection to the discipline of evolutionary psychology as a whole, some evolutionary psychologists specify only certain kinds of traits as likely to have been the result of natural selection. For example, Tooby and Cosmides argue that there are traits resulting from genetic variance with little or no adaptive significance, but that complex adaptations that are universal in a species are much more likely to have been the result of natural selection. It is not always easy to determine which traits 'must' have been the result of natural selection rather than evolutionary flukes or accidents. It is also the case that there many be numerous equally plausible accounts for the emergence of such and such a trait, and that being able to empirically verify how or why this trait must have come about is instrinsically answerable and a subject of mere speculation rather than science.
Downes, Stephen M., "Evolutionary Psychology", The Stanford Encyclopedia of Philosophy (Summer 2014 Edition), Edward N. Zalta (ed.), URL = <plato.stanford.edu/archives/sum2014/entries/evolutionary-psycholo....