At least five different theoretical approaches address the "why" question. That is, why are there individual difference. Evolutionary Psychology emphasises universals of human behavior and attempts to explain individual variability in terms of alternative adaptive strategies. Behavior genetic approaches analyse the variation in behavior in terms of the complex interplay between genetic and environmental influences. Systematic work in biological theorizing has emphasized the continuity of behavior across species and searches for the biological underpinnings of temperament and complex behavior. Social cognitive theories emphasize the importance of socialization and the effect of cognitive processes to create one's unique patterning of behavior. Traditional psychoanalytic techniques, although not a major area of current personality theory, did have an influence in the theories developed throughout much of the 20th century.
The following selections from each of these approaches are meant to be lead the interested reader to some of the current literature. These are certainly not meant to be a complete selection but are rather important readings that provide good overviews of the separate approaches. The interested reader is encouraged to first consult the overviews of personality before delving into these more specialized readings.
Some of the most interesting results of the past decade have been the demonstrations of the moderate to strong heritability of personality and ability. Using techniques of quantititative behavior genetics, it has become commonplace to show that roughly 40-60% of the variation in most personality traits have a genetic base. The most exciting and counter-intuitive findings have been that the shared environment is much less important than previously thought but that the unique environment is very important.
Genes do not code for thoughts, feelings or behavior but rather code for proteins that regulate and modulate biological systems. Although promising work has been done searching for the biological bases of individual differences it is possible to sketch out these bases only in the broadest of terms. Specific neurotransmitters and brain structures can be associated with a broad class of approach behaviors and positive affects while other neurotransmitters and structures can be associated with a similarly broad class of avoidance behaviors and negative affects. Reports relating specific alleles to specific personality traits emphasize that the broad personality traits are most likely under polygenic influence and are moderated by environmental experience.
Subtle differences in neurotransmitter availability and re-uptake vary the sensitivity of individuals to cues about their environment that predict future resource availability and external rewards and punishments. It is the way these cues are detected, atttended to, stored, and integrated with previous experiences that makes each individual unique. Current work on the bases of individual differences is concerned with understanding this delicate interplay of biological propensities with environmental opportunities and constraints as they are ultimately represented in an individualÕs information processing system. With time we can expect to increase our taxonomic and predictive power by using these causal bio-social theories of individual differences.
Dimensional representations of personality dimensions have emphasized between three to seven fundamental dimensions. These dimensions are, in turn, used to organize research across different laboratories. Although there is strong agreement about the importance of Introversion/Extroversion and Emotional Stability, Psychoticism, the third factor in the "Giant 3", is seen as being composed of Agreeableness and Conscientiousness in the "Five Factor Model". The lexical work of Goldberg and Norman and their associates emphasizes a descriptive taxonomy, while the biologically driven models of Gray and Eysenck emphasize a theoretical organization of personality traits.