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Psychologists such as B. F. Skinner used to argue that people were blank slates in the sense that almost all of our behavior was learned. Evolutionary psychologists begged to differ. Who is correct?
The blank slate idea has a long history in philosophy that goes back to Aristotle. Skinner's version draws on English philosopher John Locke who developed a theory of knowledge as formed by the association of sensory experiences (and referred to a blank sheet of paper).
Brain Development is Hard to Predict
One way of deciding between the blank slate and biological determinism is to investigate the impact of unusual events in prenatal development.
Various lines of evidence indicate that if female fetuses are exposed to excessive levels of sex hormones they grow up with masculinized behavior (1). Amongst experimental monkeys, females exhibit the male pattern of more vigorous physical activity. For humans, women who were exposed to high levels of sex hormones in the womb—because their mothers mistakenly continued to take birth control pills—more self-identified as lesbians.
In the case of androgen insensitivity, a genetic disorder, persons having a male genotype can grow up looking, and behaving exactly like women. This natural experiment indicates that exposure to androgens during development determines much of masculine appearance and behavior.
Brain function is strongly affected by exposure to stress hormones so that children growing up in stressful homes where parents fight a lot score lower on IQ tests (2).
The impact of prenatal nutrition on intelligence is well established. Researchers are finding that well-nourished mothers give birth to children who grow up not just to be taller and healthier but also more intelligent, more motivated to work hard, and more economically successful (3).
Critics of such evidence might argue that it involves something going awry in development that is of questionable relevance for people growing up under more normal circumstances. Even so, it is reasonable to assume that there is a range of variation for nutrition, stress hormones, and sex hormones within which normal development may occur. So, at the very least, we can conclude that who we are is very much affected by biological factors within the womb, contrary to blank-slate theories.
A different approach is to begin from birth and ask whether many aspects of our personality and cognitive function are already decided by our genetic heritage and biology.
What Behavior is “Loaded” at Birth
Perhaps the clearest evidence against the blank slate concept is the fact that people remain much the same throughout their lives on personality dimensions. Some of us are extroverts. Others are introverts. Some of us are physically very active whereas others are less energetic. Some of us are highly emotional in response to minor events in our lives whereas others are unperturbed.
Such personality traits are predisposed by the biology of our brains and firm evidence in favor of this view comes from the fact that these traits are strongly heritable (with genetic ancestry accounting for around half of individual differences in major personality dimensions, 4).
How our brains process information is, to some degree, predetermined by brain anatomy and physiology. Neuroscientists have developed a detailed knowledge of the functional anatomy of the brain so that damage to a particular part yields predictable functional deficits. Damage to the hippocampus causes memory problems, for example.
Moreover, information travels in predictable paths within the brain. So visual information travels from the retina to the thalamus but must reach the cortex for complex pattern recognition to be accomplished and visual perception to occur.
Despite the evidence for behavioral predispositions being present at birth, the brain itself has blank-slate-like properties (5). This phenomenon was studied most extensively for cortical cells that initially do not know what they are supposed to do but develop a rapport with neighboring cells, responding most strongly to those that stimulate them the most. This means that if a person were to lose a finger, the parts of the cortex that had represented that finger will likely start responding to input from another finger.
The inherent changeableness, or plasticity, of brain cells has been compared to a blank slate upon which sensory and motor input gets written. Sometimes, that information may get erased so that something different may be written in its place.
So Who is Right?
The historical fault lines in psychology lie between the blank-slate model of the behaviorists and the opposite extreme favored by many evolutionary psychologists.
Both extremes would appear to be wrong. We know that personality traits are strongly influenced by genotype, for example, contrary to the behaviorist perspective. Some plausible mechanisms through which this occurs have been described, such as genetic modification of the number of neurotransmitter receptors.
On the other hand, the more we discover about the brain, the more we are impressed by its capacity to respond to changing sensory inputs. So it is quite implausible that people would be born with a fully functional genetically-determined brain “program” that solved some Darwinian problems such as preventing spousal infidelity.
The brain may not be entirely blank at birth but it is not entirely programmed either. It is an interesting mix of script and improvisation.
References
1. Barber, N. (2002). The science of romance. Buffalo, NY: Prometheus.
2. Delaney-Black, V., Covington, C., Ondersma, S. J., Nordstrom-Klee, B., Templin, T., Ager, L., et al. (2002). Violence exposure, trauma, and IQ and/or reading deficits among urban children. Archives of Pediatric and Adolescent Medicine, 156, 280-285.
3. Case, A. & Paxon, C. (2008). Stature and status: Height, ability and labour market outcomes. Journal of Political Economy, 116, 491-532.
4. Plomin, R. (1990). Nature and nurture: An introduction to human behavioral genetics. Belmont, CA: Wadsworth.
5. Kalisman, N., Silberberg, G., and Markram, H. (2005). The neocortical microcircuit as a tabula rasa. Proceedings of the National Academy of Sciences, 102, 880-885.
