Although genetic influences on many aspects of human cognition are mediated and modulated by culture, one research tradition is to consider the possibility of more direct mechanistic connections of particular behavioral traits to particular genes. Such causal links, if identifiable, would once and for all establish direct genetic contributions for such traits, and also might offer prospects for interventions of the sort described in the next chapter. Initial research has focused on attempts to define and catalogue the unitary behavioral objects to be understood, and then to characterize these at multiple levels ranging from clinical description to neurological function to genetic causation.
One prominent school of thought in evolutionary psychology proposes that the human mind includes a collection of modules selectively designed to solve the most important functional problems for fitness encountered during humankind's evolutionary history. These behavioral modules may be called upon more or less individually when we find ourselves in particular predicaments.37 Such an arrangement of the mind, it is argued, can better respond to such diverse challenges as facial recognition, child rearing, adaptive social interchange, and language acquisition than would a single general-purpose cognitive arrangement.
A somewhat different possibility is that emotions such as fear, anger, lust, envy, pride, or romantic love are building blocks of human behaviors that correspond to specifc situations rather than to specifc functions. In this view, genes responsible for particular emotional tendencies have been shaped by natural selection according to circumstances routinely encountered in human evolution. Thus, a given emotion simultaneously may serve multiple functions. Fear, for example, leads to physiological arousal, avoidance and escape reactions, and communication of danger to others. Multiple emotions also unite to serve a single function, such as when fear, envy, and anger combine to prevent loss of a mate. In general, emotions are either pleasurable or painful because of the evolution-forged linkage of these perceptions with fitness-enhancing and fitness-reducing behaviors, respectively. If fear rather than sexual contact had been associated with reproduction, the former would be perceived as pleasure and the latter as pain.
Unfortunately, such broad behavioral modules or emotional descriptions have not yet been connected with detailed molecular or genetic traits. Correlational evidence of genetic involvement for such features has come from the twin studies described above, but in few instances have specifiable genes and their particular metabolic agents been implicated in trait expression at these general levels.
The fields of medical and clinical psychology have provided other compartmentalized descriptions of human behavior for which molecular or cellular causation can be sought. Pronounced mental disorders provide among the clearest examples.
Schizophrenia, the most common psychosis, involves disorders of the thinking process that lead to delusions and hallucinations, paranoia, and withdrawal of the individual from other people and the outside world. Genetic influence is suggested by the fact that relatives of schizophrenics are at several-fold greater risk for the disease (even when raised separately), and that an identical twin of a schizophrenic is twice as likely as a fraternal twin to display the disorder. Evolutionary speculation has centered on why schizophrenia is so common in diverse societies worldwide (about 1 percent of the population), and whether it may have had some selective advantage.38 However, a clear understanding has yet to emerge. Because of the diverse clinical expressions of schizophrenia and the likelihood that these are due to different kinds of brain damage, schizophrenia now is considered to be a group of mental disorders rather than a single entity. Thus, any genetic contributions to "schizophrenia" as broadly defined probably will not be uniform or simple.
The brain damage responsible for many neurological disorders often is environmental in origin, but the mere fact that such deficits can be localized neurologically suggests that even the most subtle of human perceptual capabilities may be studied at cellular and molecular levels. Prosopagnosia is a neurological condition in which patients can describe human faces with accuracy, but remain unable to recognize at sight even close friends. At least two behav-
iorally and clinically distinct types of prosopagnosia are known, stemming from lesions in different areas of the brain. Another condition that sometimes entails a specific deficit in brain function is dyslexia, a neurological disorder first noticed as reading and language difficulties in children. A biological marker for developmental dyslexia recently was identified in the form of distinctive brain activity patterns localized to a small area of cerebral cortex.39 Anosognosia is another example of a condition entailing a specific deficit in brain function, in this case involving damage to the right parietal lobe. Patients are paralyzed on the left side of their bodies but steadfastly refuse to acknowledge the disability.40
Disruptions of neurological function occasionally may arise from genetic mutations also. Some forms of dyslexia someday may provide empirical examples because they tend to run in families. The term genocopy has been suggested for any genetic effect that mimics a more widespread phenotypic condition induced by the environment.41 The word is a reverse counterpart to the more commonly employed term phenocopy, an environmental effect that mimics a genetic condition.
Autism or mindblindness is a devastating neurological disorder clinically diagnosable by the age of three.42 Autism typically is associated with moderate mental retardation, but some autistic children have normal or superior IQs.43 What all autistic people share is an obliviousness to other people's thoughts and feelings. By five years of age, most nonautistic children understand that brains are for thinking and dreaming, but autistic children never develop a working concept of their own or other people's minds; they tend to relate to others as opaque, alien beings. Although the behavioral development of autism in infants and children is described reasonably well, neither structural neurological deficits nor genetic alterations have as yet been associated with the disorder. One possibility is that normal genetic operations or cellular factors are disrupted at critical times during an individual's development, but only transiently. Certainly, many developmental genes normally switch on or off at critical times and might malfunction only fleetingly, with nonetheless devastating and continuing consequences.
In the past century, medical researchers have identified more than a thousand neurological syndromes that are inherited through families in relatively straightforward Mendelian fashion.44 Examples include Batten disease, Wolf-Hirschhorn syndrome, certain forms of Alzheimer disease, Huntington disease, and various other neurological disorders. Researchers now are beginning to home in on the particular genes involved in various conditions of the brain (see accompanying box). It is not surprising that neurological disorders often involve mental illnesses,45 nor that major disabilities generally
Examples of brain disorders for which researchers discovered contributing genes during the year 1997
Epilepsy and mental retardation. A mutant gene on the X chromosome can produce these disorders in women. Male carriers of the mutation do not display the effects but can pass the condition to daughters.
Heroin addiction. Some cases have been associated with particular forms of a gene on chromosome 11 that codes for a dopamine receptor on brain cells. This may be the same gene that influences "novelty-seeking" behavior (see the text).
Parkinson disease (PD). A mutant gene was identified on chromosome 4 that produces misfolded proteins. As the latter accumulate in the brain, neurons die, neurotransmission is compromised, and tremors and muscle rigidity ensue that are the hallmark of some early onset cases of PD.
Boxer brain. A particular form of a gene on chromosome 19 involved with the deposition of a brain protein (beta-amyloid) appears to increase the likelihood that boxers display chronic brain injury from repeated blows to the head.
Obsessive-compulsive disorder (OCD). A gene on chromosome 22 was identified that encodes an enzyme that breaks down excess neurotransmitters in the brain. Forms of this gene that yield less active enzyme appear to be associated with some cases of OCD in men. The disorder, experienced by 1-3 percent of the population, is characterized by a pathological compulsion to repeat certain behaviors, such as washing hands.
Source: After J. Glausiusz in Discover 38 (1997): 38.
lend themselves to clearer diagnosis and characterization. More subtle genetic, chemical, and cellular influences on brain function and the mind's "normal" operation are more difficult to pinpoint, yet in the long run should prove to be the most interesting.
Recent reports claim to have identified the first specific gene involved in neurotransmission that leads to variation in a normal human personality trait: novelty seeking.46 The polymorphic gene (D4DR) encodes a receptor protein that at the molecular level binds neurotransmitters related to dopamine. At the behavioral level, the gene appears to influence how quick-tempered, excitable, impulsive, and extravagant a person may be. The gene was identified, and the gene-personality link originally sought, because of the known role of D4DR in general neurotransmission. The findings were correlational in the sense that they merely demonstrate an association between alternative forms of the gene and psychometric scores on personality tests. Nevertheless, this research suggests that standard variations in personality traits and emotions may be influenced by specifiable genes for molecular modulators of brain function.
Another example is provided by a recently discovered "chronic anxiety" gene that naturally regulates production of the brain chemical serotonin47—the same substance medically targeted by the antidepressant drug Prozac. Reportedly, this gene by itself accounts for about 4 percent of the variation among humans in degree of neuroticism. Subtle genetic influences of this sort might be pervasive. Certainly, a plethora of natural (and artificially synthesized) neurochemicals that influence brain operations and human behaviors are known to medicine and pharmacology.48
The human brain is a dynamic network composed of about 100 billion neurons, capable of 100 trillion different connections. About 30,000 to 50,000 human genes are expressed (present as messenger RNAs or proteins) in this organ. A new and rapidly growing enterprise in neurological genetics involves isolating, these transcribed or translated molecules in the brain, and then working backward to identify the genes themselves. To what extent this and similar approaches will expand significantly our knowledge of the mind's operation remains to be seen. Psychologists are beginning to ask whether psychology ever will be the same after the genome is mapped.49 Sir Francis Crick, a codiscoverer of the double-helical structure of DNA, has gone so far as to urge that the neurosciences should extend into a search for the soul itself!50
The recent enthusiasm for neurogenetic determinism has hardly gone unchallenged, nor should it. Neurogenetic research during the "decade of the brain" has not yet yielded truly profound results. Neurobiology, psychiatry, and psychology are coming into extensive contact with the evolutionary-genetic sciences for the first time, and must completely reorient themselves. Some recent writings seem naively unaware, for example, of previous nature/nurture battles on related fronts and the hard-won lessons to be learned from them. Any synthetic understanding of the mind's operations and of human behavior will not come from myopically reduction-istic approaches that neglect multiple levels of biological, personal, and social causation. However, neither will such understanding come from uncritical holistic approaches alone. As in many areas of biological science, communication among disparate specialties has become crucial to solving the remaining mysteries of the mind.
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