Evolutionary psychology
Evolutionary psychology (abbreviated EP) is a theoretical approach to psychology that attempts to explain mental and psychological traitssuch as memory, perception, or languageas adaptations, i.e., as the functional products of natural selection. The purpose of this approach is to bring an adaptationist way of thinking about biological mechanisms such as the immune system into the field of psychology, and to approach psychological mechanisms in a similar way.  In short, evolutionary psychology is focused on how evolution has shaped the mind and behavior. Though applicable to any organism with a nervous system, most research in evolutionary psychology focuses on humans.  Closely related fields are human behavioral ecology, dual inheritance theory, and sociobiology.

# Overview
Evolutionary psychology sounds as if it were merely another branch of traditional normative psychology.  This is not the case.  To use its own terminology, EP seeks to heal a fundamental division at the very heart of science --- that between the "soft" human social sciences and the "hard" natural sciences.  That human science is a category separate and apart from natural science ultimately carries with it a wrongheaded philosophical implication that humans are, in some way, separate and apart from the natural world ---the historical fallacy that the Darwinian perspective now works to correct.  The fact that human beings are living organisms demands that psychology, as the study of the human mind, be understood as a branch of biology, the science of all living organisms.  And, in keeping with the requirements of good biological science, EP derives its theoretical foundation solely from the same source as biology:  Darwinian evolutionary theory.  EP also properly understands its place within the rubric of biology as a branch of primatology.  Succinctly, EP is the natural science studying the mind of the human primate.  Thus, in the broadest sense, EP is the rather dramatic announcement to the world that psychology has finally "graduated" to the status of natural science and that the fundamental dichotomy at the heart of science between human science and natural science is now (or will soon be) at an end.  With the birth of EP, there will, eventually, be needed only the umbrella term "science" to represent one fully consistent  "monolith" of falsifiable knowledge.  Likewise, the phrase "evolutionary psychology" is seen as merely a stopgap terminology that will end upon the full recognition that psychology is, necessarily, a biological science.  In the future, the study of human psychology will be completely transformed by the Darwinian approach...it wont be called Evolutionary Psychology.  It will just be called psychology".
Just as human evolutionary physiology has worked to identify physical adaptations of the body that represent "human physiological nature," the purpose of evolutionary psychology is to identify evolved emotional and cognitive adaptations that represent "human psychological nature."  EP is, to quote Steven Pinker, "not a single theory but a large set of hypotheses" and a term which "has also come to refer to a particular way of applying evolutionary theory to the mind, with an emphasis on adaptation, gene-level selection, and modularity." EP proposes that the human brain comprises many functional mechanisms, called psychological adaptations or evolved cognitive mechanisms or cognitive modules designed by the process of natural selection. Examples include language acquisition modules, incest avoidance mechanisms, cheater detection mechanisms, intelligence and sex-specific mating preferences, foraging mechanisms, alliance-tracking mechanisms, agent detection mechanisms, and so on. 
EP has roots in cognitive psychology and evolutionary biology (See also sociobiology).  It also draws on behavioral ecology, artificial intelligence, genetics, ethology, anthropology, archaeology, biology, and zoology. EP is closely linked to sociobiology, but there are key differences between them including the emphasis on domain-specific rather than domain-general mechanisms, the relevance of measures of current fitness, the importance of mismatch theory, and psychology rather than behaviour. Many evolutionary psychologists, however, argue that the mind consists of both domain-specific and domain-general mechanisms, especially evolutionary developmental psychologists. Most sociobiological research is now conducted in the field of behavioral ecology.
The term evolutionary psychology was probably coined by Michael Ghiselin in his 1973 article in Science. Jerome Barkow, Leda Cosmides and John Tooby popularized the term "evolutionary psychology" in their highly influential 1992 book The Adapted Mind: Evolutionary Psychology and The Generation of Culture. EP has been applied to the study of many fields, including economics, aggression, law, psychiatry, politics, literature, and sex.
EP uses Nikolaas Tinbergen's four categories of questions and explanations of animal behavior.  Two categories are at the species level; two, at the individual level.  The species-level categories (often called ultimate explanations) are
- the function (i.e., adaptation) that a behavior serves and
- the evolutionary process (i.e., phylogeny) that resulted in this functionality.
The individual-level categories are
- the development of the individual (i.e., ontogeny) and
- the proximate mechanism (e.g., brain anatomy and hormones).
Evolutionary psychology emphasizes the functionality category.

## Principles of evolutionary psychology
Evolutionary psychology is a hybrid discipline that draws insights from modern evolutionary theory, biology, cognitive psychology, anthropology, economics, computer science, and paleoarchaeology. The discipline rests on a foundation of core premises. According to David Buss, a prominent researcher in the field, these include:
- Manifest behavior depends on underlying psychological mechanisms, information processing devices housed in the brain, in conjunction with the external and internal inputs that trigger their activation.
- Evolution by selection is the only known causal process capable of creating such complex organic mechanisms.
- Evolved psychological mechanisms are functionally specialized to solve adaptive problems that recurred for humans over deep evolutionary time.
- Selection designed the information processing of many evolved psychological mechanisms to be adaptively influenced by specific classes of information from the environment.
- Human psychology consists of a large number of functionally specialized evolved mechanisms, each sensitive to particular forms of contextual input, that get combined, coordinated, and integrated with each other to produce manifest behavior.
Similarly, Leda Cosmides and John Tooby, two of the founders of the field, offer these five foundational principles of evolutionary psychology:
- The brain is a physical system. It functions as a computer. Its circuits are designed to generate behavior that is appropriate to your environmental circumstances.
- Our neural circuits were designed by natural selection to solve problems that our ancestors faced during our species' evolutionary history.
- Consciousness is just the tip of the iceberg; most of what goes on in your mind is hidden from you. As a result, your conscious experience can mislead you into thinking that our circuitry is simpler than it really is. Most problems that you experience as easy to solve are very difficult to solve -- they require very complicated neural circuitry
- Different neural circuits are specialized for solving different adaptive problems.
- Our modern skulls house a stone age mind.

# General evolutionary theory
William Paley, drawing upon the work of many others, argued that organisms are machines designed to function in particular environments. Paley believed that this evidence of 'design' was evidence for a designerGod. In direct contrast, Richard Dawkins argues that evolution consistently destroys those organisms who cannot adapt to environmental changes, their niches being filled by competing organisms who can exploit the new changes; thus, there is no designer-God making perfect machines. Charles Darwin opposed Paley's argument that organisms are designed for particular environments by invoking both Adam Smith and Malthus. He asserted that the supposed good design of organisms to environments was an accidental result of natural selection. The theory of natural selection, formulated in depth by Charles Darwin and Alfred Russel Wallace, provided a scientific account of the origins of functional design in the natural world.
Evolutionary psychology is rooted in evolutionary theory. It is sometimes seen not simply as a sub-discipline of psychology but as a way in which evolutionary theory can be used as a meta-theoretical framework within which to examine the entire field of psychology, however many evolutionary biologists challenge the basic evolutionary premises of evolutionary psychology.
Natural selection, a key component of evolutionary theory, involves three main ingredients:
- Genetically based inheritance of traits - some traits are passed down from parents to offspring in genes,
- Variation - heritable traits vary within a population (now we know that mutation is the source of this genetic variation),
- Differential survival and reproduction - these traits will vary in how strongly they promote the survival and reproduction of their bearers.
Selection refers to the process by which environmental conditions "select" organisms with the appropriate traits to survive; these organisms will have such traits more strongly represented in the next generation. This is the basis of adaptive evolution. Darwin's great claim was that this "natural selection" was creative - it could lead to new traits and even new species, it was centred on individual survival, and it could explain the broad scale patterns of evolution.
Many traits that are selected for can actually hinder survival of the organism while increasing its reproductive opportunities. Consider the classic example of the peacock's tail. It is metabolically costly, cumbersome, and essentially a "predator magnet." What the peacock's tail does do is attract mates. Thus, the type of selective process that is involved here is what Darwin called "sexual selection." Sexual selection can be divided into two types:
- Intersexual selection, which refers to the traits that one sex generally prefers in the other sex, (e.g. the peacock's tail).
- Intrasexual competition, which refers to the competition among members of the same sex for mating access to the opposite sex, (e.g. two stags locking antlers).

### Inclusive fitness
Inclusive fitness theory, which was proposed by William D. Hamilton in 1964 as a revision to evolutionary theory, is basically a combination of natural selection, sexual selection, and kin selection. It refers to the sum of an individual's own reproductive success plus the effects the individual's actions have on the reproductive success of their genetic relatives. General evolutionary theory, in its modern form, is essentially inclusive fitness theory.
Inclusive fitness theory resolved the issue of how "altruism" evolved. The dominant, pre-Hamiltonian view was that altruism evolved via group selection: the notion that altruism evolved for the benefit of the group. The problem with this was that if one organism in a group incurred any fitness costs on itself for the benefit of others in the group, (i.e. acted "altruistically"), then that organism would reduce its own ability to survive and/or reproduce, therefore reducing its chances of passing on its altruistic traits. Furthermore, the organism that benefited from that altruistic act and only acted on behalf of its own fitness would increase its own chance of survival and/or reproduction, thus increasing its chances of passing on its "selfish" traits.
Inclusive fitness resolved "the problem of altruism" by demonstrating that altruism can evolve via kin selection as expressed in Hamilton's rule:
In other words, altruism can evolve as long as the fitness cost of the altruistic act on the part of the actor is less than the degree of genetic relatedness of the recipient times the fitness benefit to that recipient. 
This perspective reflects what is referred to as the gene-centered view of evolution and demonstrates that group selection is a very weak selective force. However, in recent years group selection has been making a comeback, (albeit a controversial one), as multilevel selection, which posits that evolution can act on many levels of functional organization, (including the "group" level), and not just the "gene" level.

### Overview of some foundational ideas related to evolutionary psychology
Table from Mills, M.E. (2004). Evolution and motivation. Symposium paper presented at the Western Psychological Association Conference, Phoenix, AZ. April, 2004.

# Middle-level evolutionary theories
Middle-level evolutionary theories are theories that encompass broad domains of functioning. They are compatible with general evolutionary theory but not derived from it. Furthermore, they are applicable across species. During the early 1970s, three very  important middle-level evolutionary theories were contributed by Robert Trivers:
- The theory of reciprocal altruism explains how altruism can arise amongst non-kin, as long as there is a sufficient probability of the recipient of the altruistic act reciprocating at a later date. The possibility was also noted by Trivers, later coined 'indirect altruism' by Richard Alexander, that reciprocation could be provided by third parties, raising the issue of social reputation. These theories have been criticized as example of "Just So stories" within evolutionary biology.
- Parental investment theory refers to the different levels of investment in offspring on the part of each sex. For example, females in any species are defined as the sex with the larger gamete. In humans, females release approximately one large, metabolically costly egg per month, as opposed to the millions of relatively tiny and metabolically cheap sperm that are produced each day by males. Females are fertile for only a few days each month, while males are fertile every day of the month. Females also have a nine month gestation period, followed by a few years of lactation. Males' obligatory biological investment can be achieved with one copulatory act. Consequently, human females have a significantly higher obligatory investment in offspring than males do. (In some species, the opposite is true.)  Because of this difference in parental investment between males and females, the sexes face different adaptive problems in the domains of mating and parenting. Therefore, it is predicted that the higher investing sex will be more selective in mating, and the lesser investing sex will be more competitive for access to mates. Thus, differences in behaviour between sexes is predicted to exist not because of maleness or femaleness per se, but because of different levels of parental investment.
- The theory of parent-offspring conflict rests on the fact that even though a parent and his/her offspring are 50% genetically related, they are also 50% genetically different. All things being equal, a parent would want to allocate their resources equally amongst their offspring, while each offspring may want a little more for themselves. Furthermore, an offspring may want a little more resources from the parent than the parent is willing to give. In essence, parent-offspring conflict refers to a conflict of adaptive interests between parent and offspring. However, if all things are not equal, a parent may engage in discriminative investment towards one sex or the other, depending on the parent's condition.
Additional middle-level evolutionary theories used in EP include:
- The Trivers-Willard hypothesis, which proposes that parents should invest more in the sex that gives them the greatest reproductive payoff (grandchildren) with increasing or marginal investment. Females are the heavier parental investors in our species. Because of that, females have a better chance of reproducing at least once in comparison to males. Thus, according to the Trivers-Willard hypothesis, parents in good condition are predicted to favor investment in sons, and parents in poor condition are predicted to favor investment in daughters.
- r/K selection theory, which, in ecology, relates to the selection of traits in organisms that allow success in particular environments. r-selected species, (in unstable or unpredictable environments), produce many offspring, each of which is unlikely to survive to adulthood, while K-selected species, (in stable or predictable environments), invest more heavily in fewer offspring, each of which has a better chance of surviving to adulthood.
- Evolutionary game theory, the application of population genetics-inspired models of change in gene frequency in populations to game theory.
- Evolutionary stable strategy, which refers to a strategy, which if adopted by a population, cannot be invaded by any competing alternative strategy.

# Evolved psychological mechanisms
Evolutionary psychology is based on the belief that, just like hearts, lungs, livers, kidneys, and immune systems, cognition has functional structure that has a genetic basis, and therefore has evolved by natural selection. Like other organs and tissues, this functional structure should be universally shared amongst a species, and should solve important problems of survival and reproduction. Evolutionary psychologists seek to understand psychological mechanisms by understanding the survival and reproductive functions they might have served over the course of evolutionary history.
Evolutionary psychologists subdivide the concept of psychological mechanisms into two general categories:
- Domain-specific mechanisms, which deal with recurrent adaptive problems over the course of human evolutionary history
- Domain-general mechanisms, which deal with evolutionary novelty

# Environment of evolutionary adaptedness
The term environment of evolutionary adaptedness, often abbreviated EEA, was coined by John Bowlby as part of attachment theory. It refers to the environment to which a particular evolved mechanism is adapted.  More specifically, the EEA is defined as the set of historically recurring selection pressures that formed a given adaptation, as well as those aspects of the environment that were necessary for the proper development and functioning of the adaptation.  In the environment in which ducks evolved, for example, attachment of ducklings to their mother had great survival value for the ducklings.  Because the first moving being that a duckling was likely to see was its mother, a psychological mechanism that evolved to form an attachment to the first moving being would therefore properly function to form an attachment to the mother. In novel environments, however, the mechanism can malfunction by forming an attachment to a dog or human instead.
The genus Homo, which includes modern humans, appeared between 1.5 and 2.5 million years ago, a time that roughly coincides with the start of the Pleistocene 1.8 million years ago. Because the Pleistocene ended a mere 12,000 years ago, most human adaptations either newly evolved during the Pleistocene, or were maintained by stabilizing selection during the Pleistocene.  Evolutionary psychology therefore proposes that the majority of human psychological mechanisms are adapted to reproductive problems frequently encountered in Pleistocene environments. In broad terms, these problems include those of growth, development, differentiation, maintenance, mating, parenting, and social relationships. To properly understand human mating psychology, for example, it is essential to recognize that in the EEA (as now) women got pregnant and men did not.
If humans are mostly adapted to Pleistocene environments, then some psychological mechanisms should occasionally exhibit mismatches to the modern environment, similar to the attachment patterns of ducks. One example is the fact that although about 10,000 people are killed with guns in the US annually, whereas spiders and snakes kill only a handful, people nonetheless learn to fear spiders and snakes about as easily as they do a pointed gun, and more easily than an unpointed gun, rabbits or flowers. A potential explanation is that spiders and snakes were a threat to human ancestors throughout the Pleistocene, whereas guns, rabbits and flowers were not. There is thus a mismatch between our evolved fear learning psychology and the modern environment.
In sum, evolutionary psychology argues that to properly understand the functions of the brain one must understand the properties of the environment in which the brain evolved.

# Controversies
The application of evolutionary theory to animal behavior is uncontroversial. However, adaptationist approaches to human psychology are contentious, with critics questioning the scientific nature of evolutionary psychology, and with more minor debates within the field itself. The history of debate from the evolutionary psychology perspective is covered in detail by Segerstrale (2000) and Alcock (2001); also see a recent overview of EP with rebuttals to critics in Tooby, J. & Cosmides, L. (2005). Conceptual foundations of evolutionary psychology.

# Notes
-  Evans and Zarate 2005, 169
-  evolutionary psychology Psyche Games. Accessed August 22 2007
-  Seltin, Melissa. (August 1988) The Evolution of Evolutionary Psychology: From Sociobiology to Evolutionary Psychology Accessed August 22 2007
-  00265 Behavioral Ecology and Sociobiology Accessed August 22 2007
-  Jump up to: 5.0 5.1 Cosmides, Leda and John Tooby. January 13 1997 Evolutionary Psychology: A Primer Accessed August 222007
-  See for examples, Gould, S.J. (2002) The Structure of Evolutionary Theory
-  CDC pdf
-  Ohman and Mineka 2001
-  Tooby, J. & Cosmides, L. (2005). Conceptual foundations of evolutionary psychology. In D. M. Buss (Ed.), The Handbook of Evolutionary Psychology (pp. 5-67). Hoboken, NJ: Wiley. Full text