LECTURE 10: CHANGE OF MEASURE AND THEGIRSANOV THEOREM

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LECTURE 10: CHANGE OF MEASURE AND THE GIRSANOV THEOREM 1. Introduction The Cameron-Martin theorem, which has figured prominently in the developments of the last several lectures, is the most important special case of the far more general Girsanov theorem, which is our next topic of discussion. Like the Cameron-Martin theorem, the Girsanov theorem relates the Wiener measure P to different probability measures Q on the space of continuous paths by giving an explicit formula for the likelihood ratios between them.
  • sterling investors
  • drift term
  • pricing barrier options
  • exchange rate yt
  • girsanov theorem
  • change of measure
  • likelihood ratio
  • probability
  • exp
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Pinker.qxd 09/29/04 2:54 PM Page 1
Steven Pinker
Why nature & nurture
won’t go away
W hen Richard Mulcaster referred in that debates over nature and nurture
1581 to “that treasure . . . bestowed on evoke more rancor than just about any
them by nature, to be bettered in them issue in the world of ideas.
by nurture,” he gave the world a eupho- During much of the twentieth century,
nious name for an opposition that has a common position in this debate was to
been debated ever since. People’s beliefs deny that human nature existed at all–
about the relative importance of heredi- to aver, with José Ortega y Gasset, that
ty and environment affect their opinions “Man has no nature; what he has is his-
on an astonishing range of topics. Do tory.” The doctrine that the mind is a
adolescents engage in violence because blank slate was not only a cornerstone
of the way their parents treated them of behaviorism in psychology and social
early in life? Are people inherently ag- constructionism in the social sciences,
gressive and sel½sh, calling for a market but also extended widely into main-
1economy and a strong police, or could stream intellectual life.
they become peaceable and cooperative, Part of the blank slate’s appeal came
allowing the state to wither and a spon- from the realization that many differ-
taneous socialism to blossom? Is there a ences among people in different classes
universal aesthetic that allows great art and ethnic groups that formerly were
to transcend time and place, or are peo- 1 Carl N. Degler, In Search of Human Nature:
ple’s tastes determined by their era and The Decline and Revival of Darwinism in American
culture? With so much seemingly at Social Thought (New York: Oxford University
Press, 1991); Steven Pinker, The Blank Slate: stake in so many ½elds, it is no surprise
The Modern Denial of Human Nature (New York:
Viking, 2002); Robin Fox, The Search for Soci-
ety: Quest for a Biosocial Science and Morality
Steven Pinker, Johnstone Family Professor in the (New Brunswick, N.J.: Rutgers University
department of psychology at Harvard University, Press, 1989); Eric M. Gander, On Our Minds:
conducts research on language and cognition. A How Evolutionary Psychology Is Reshaping the
Nature-Versus-Nurture Debate (Baltimore: Fellow of the American Academy since 1998, he
Johns Hopkins University Press, 2003); Johnis the author of six books, including “How the
Tooby and Leda Cosmides, “The Psychological
Mind Works” (1997), “The Language Instinct”
Foundations of Culture,” in The Adapted Mind:
(2000), and “The Blank Slate” (2002). Evolutionary Psychology and the Generation of
Culture, ed. Jerome H. Barkow, Leda Cosmides,
and John Tooby (New York: Oxford University© 2004 by the American Academy of Arts
Press, 1992).& Sciences
Dædalus Fall 2004 1Pinker.qxd 09/29/04 2:54 PM Page 2
Steven thought to reflect innate disparities in that infants have a precocious grasp
Pinker talent or temperament could vanish of objects, intentions, numbers, faces,
on
through immigration, social mobility, tools, and language. Behavioral geneticshuman
nature and cultural change. But another part has shown that temperament emerges
of its appeal was political and moral. If early in life and remains fairly constant
nothing in the mind is innate, then dif- throughout the life span, that much of
ferences among races, sexes, and classes the variation among people within a cul-
can never be innate, making the blank ture comes from differences in genes,
slate the ultimate safeguard against rac- and that in some cases particular genes
ism, sexism, and class prejudice. Also, can be tied to aspects of cognition, lan-
the doctrine ruled out the possibility guage, and personality. Neuroscience
that ignoble traits such as greed, preju- has shown that the genome contains a
dice, and aggression spring from human rich tool kit of growth factors, axon
nature, and thus held out the hope of un- guidance molecules, and cell adhesion
limited social progress. molecules that help structure the brain
Though human nature has been debat- during development, as well as mecha-
ed for as long as people have pondered nisms of plasticity that make learning
their condition, it was inevitable that the possible.
debate would be transformed by the re- These discoveries not only have shown
cent efflorescence of the sciences of that the innate organization of the brain
mind, brain, genes, and evolution. One cannot be ignored, but have also helped
outcome has been to make the doctrine to reframe our very conception of nature
2of the blank slate untenable. No one, and nurture.
of course, can deny the importance of
learning and culture in all aspects of Nature and nurture, of course, are not
human life. But cognitive science has alternatives. Learning itself must be
shown that there must be complex in- accomplished by innate circuitry, and
nate mechanisms for learning and cul- what is innate is not a set of rigid in-
ture to be possible in the ½rst place. Evo- structions for behavior but rather pro-
lutionary psychology has documented grams that take in information from the
hundreds of universals that cut across senses and give rise to new thoughts and
the world’s cultures, and has shown that actions. Language is a paradigm case:
many psychological traits (such as our though particular languages such as Jap-
taste for fatty foods, social status, and anese and Yoruba are not innate, the ca-
risky sexual liaisons) are better adapted pacity to acquire languages is a uniquely
to the evolutionary demands of an an- human talent. And once acquired, a lan-
cestral environment than to the actual guage is not a ½xed list of sentences, but
demands of the current environment. a combinatorial algorithm allowing an
Developmental psychology has shown in½nite number of new thoughts to be
expressed.2 Pinker, The Blank Slate; Gary F. Marcus, The
Moreover, because the mind is a com-Birth of the Mind: How a Tiny Number of Genes
Creates the Complexities of Human Thought (New plex system composed of many inter-
York: Basic Books, 2004); Matt Ridley, Nature acting parts, it makes no sense to ask
Via Nurture: Genes, Experience, and What Makes whether humans are sel½sh or generous
Us Human (London: Fourth Estate, 2003);
or nasty or noble across the board. Rath-Robert Plomin, Michael J. Owen, and Peter
er, they are driven by competing motivesMcGuf½n, “The Genetic Basis of Complex Hu-
man Behaviors,” Science 264 (1994): 1733–1739. elicited in different circumstances. And
2 Dædalus Fall 2004Pinker.qxd 09/29/04 2:54 PM Page 3
if genes affect behavior, it is not by tug- there is a widespread desire that the Why nature
& nurtureging on the muscles directly, but by their whole issue would somehow just go
won’t gointricate effects on the circuitry of a away. A common position on nature and away
growing brain. nurture among contemporary scientists
Finally, questions of what people in- can be summarized as follows:
nately have in common must be distin-
No one today believes that the mind is a
guished from questions of how races,
blank slate; to refute such a belief is to tip
sexes, or individuals innately differ. Evo-
over a straw man. All behavior is the prod-
lutionary biology gives reasons to be-
uct of an inextricable interaction between
lieve that there are systematic species-
heredity and environment during develop-
wide universals, circumscribed ways in
ment, so the answer to all nature-nurture
which the sexes differ, random quantita-
questions is “some of each.” If people only
tive variation among individuals, and
recognized this truism, the political re-
few if any differences among races and
criminations could be avoided. Moreover,3ethnic groups.
modern biology has made the very dis-
This reframing of human nature also
tinction between nature and nurture ob-
offers a rational way to address the polit-
solete. Since a given set of genes can have4ical and moral fears of human nature.
different effects in different environ-
Political equality, for example, does not
ments, there may always be an environ-
hinge on a dogma that people are innate-
ment in which a supposed effect of the
ly indistinguishable, but on a commit-
genes can be reversed or canceled; there-
ment to treat them as individuals in
fore the genes impose no signi½cant con-
spheres such as education and the crim-
straints on behavior. Indeed, genes are
inal justice system. Social progress does
expressed in response to environmental
not require that the mind be free of ig-
signals, so it is meaningless to try to dis-
noble motives, only that it have other
tinguish genes and environments; doing
motives (such as the emotion of empa-
so only gets in the way of productive re-
thy and cognitive faculties that can
search.
learn from history) that can counteract
them. The attitude is often marked by words
like ‘interactionist,’ ‘developmentalist,’
‘dialectic,’ ‘constructivist,’ and ‘epige-By now most scientists reject both the
netic,’ and is typically accompanied nineteenth-century doctrine that biolo-
by a diagram with the labels ‘genes,’gy is destiny and the twentieth-century
‘behavior,’ ‘prenatal environment,’ ‘bio-doctrine that the mind is a blank slate.
chemical environment,’ ‘family environ-At the same time, many express a dis-
ment,’ ‘school environment,’ ‘culturalcomfort with any attempt to character-
environment,’ and ‘socioeconomic envi-ize the innate organization that the mind
ronment,’ and arrows pointing fromdoes have (even in service of a better
every label to every other label. understanding of learning). Instead,
This doctrine, which I will call holistic
3 John Tooby and Leda Cosmides, “On the interactionism, has considerable appeal.
Universality of Human Nature and the Unique- It is based on some unexceptionable
ness of the Individual: The Role of Genetics points, such as that nature and nurture
and Adaptation,” Journal of Personality 58 are not mutually exclusive, that genes
(1990): 17–67.
cannot cause behavior directly, and that
the direction of causation can go both4 Pinker, The Blank Slate.
Dædalus Fall 2004 3Pinker.qxd 09/29/04 2:54 PM Page 4
Steven ways (for example, school can make you his culture, from the man-made part
Pinker smarter, and smart people are most en- of the environment, from other humanon 6gaged by schooling). It has a veneer of beings.” Postmodernism and socialhuman
nature moderation, of conceptual sophistica- constructionism, which dominate many
tion, and of biological up-to-dateness. of the humanities, vigorously assert that
And as John Tooby and Leda Cosmides human emotions, conceptual categories,
have put it, it promises “safe conduct and patterns of behavior (such as those
across the politicized mine½eld of mod- characterizing men and women or ho-
5ern academic life.” mosexuals and heterosexuals) are social
But the very things that make holistic constructions. Even many humanists
interactionism so appealing should also who are not postmodernists insist bio-
make us wary of it. No matter how com- logy can provide no insight into human
plex an interaction is, it can be under- mind and behavior. The critic Louis
stood only by identifying the compo- Menand, for instance, recently wrote
nents and how they interact. Holistic that “every aspect of life has a biological
interactionism can stand in the way of foundation in exactly the same sense,
such understanding by dismissing any which is that unless it was biologically
attempt to disentangle heredity and en- possible it wouldn’t exist. After that, it’s
7vironment as uncouth. As Dan Dennett up for grabs.”
has satirized the attitude: “Surely ‘every- Nor is a belief in the blank slate absent
one knows’ that the nature-nurture de- among prominent scientists. Richard
bate was resolved long ago, and neither Lewontin, Leon Kamin, and Steven
side wins since everything-is-a-mixture- Rose, in a book entitled Not in Our Genes,
of-both-and-it’s-all-very-complicated, asserted that “the only sensible thing to
so let’s think of something else, right?” say about human nature is that it is ‘in’
In the following pages I will analyze that nature to construct its own his-
8the tenets of holistic interactionism and tory.” Stephen Jay Gould wrote that
show that they are not as reasonable or the “brain [is] capable of a full range of
9as obvious as they ½rst appear. behaviors and predisposed to none.”
Anne Fausto-Sterling expressed a com-
mon view of the origin of sex differ-No one believes in the extreme nurture“
ences: “The key biological fact is thatposition that the mind is a blank slate.”
boys and girls have different genitalia,Whether or not this is true among scien-
tists, it is far from true in the rest of in-
tellectual life. The prominent anthropol- 6 Ashley Montagu, ed., Man and Aggression 2nd
ogist Ashley Montagu, summing up a ed. (New York: Oxford University Press, 1973).
common understanding in twentieth-
7 Louis Menand, “What Comes Naturally,” Thecentury social science, wrote in 1973 that
New Yorker 25 November 2002.“With the exception of the instinctoid
reactions in infants to sudden with- 8 R. C. Lewontin, Steven Rose, and Leon J.
drawals of support and to sudden loud Kamin, Not in Our Genes: Biology, Ideology, and
Human Nature (New York: Pantheon Books,noises, the human being is entirely in-
1984).stinctless . . . .Man is man because he has
no instincts, because everything he is
9 Stephen Jay Gould, “Biological Potential vs.
and has become he has learned . . . from Biological Determinism,” in Ever Since Darwin:
Reflections in Natural History, ed. Stephen Jay5 Tooby and Cosmides, “The Psychological
Gould (New York: Norton, 1977).Foundations of Culture.”
4 Dædalus Fall 2004Pinker.qxd 09/29/04 2:54 PM Page 5
Why nature and it is this biological difference that are women is less than 50 percent, are
& nurtureleads adults to interact differently with attributed entirely to prejudice and hid-
won’t go
different babies whom we conveniently den barriers. The possibility that, on away
color-code in pink or blue to make it average, women might be less interested
unnecessary to go peering into their dia- than men in people-free pursuits is simi-
10 13pers for information about gender.” larly unspeakable. The point is not that
These opinions spill into research and we know that evolution or genetics are
policy. Much of the scienti½c consensus relevant to explaining these phenomena,
on parenting, for example, is based on but that the very possibility is often
studies that ½nd a correlation between treated as an unmentionable taboo rath-
the behavior of parents and the behavior er than as a testable hypothesis.
of children. Parents who spank have
children who are more violent; authori- For every question about nature and“
tative parents (neither too permissive nurture, the correct answer is ‘some of
nor too punitive) have well-behaved each.’” Not true. Why do people in Eng-
children; parents who talk more to their land speak English and people in Japan
children have children with better lan- speak Japanese? The ‘reasonable com-
guage skills. Virtually everyone con- promise’ would be that the people in
cludes that the behavior of the parent England have genes that make it easier
causes the outcomes in the child. The to learn English and the people in Japan
possibility that the correlations may have genes that make it easier to learn
arise from shared genes is usually not Japanese, but that both groups must be
11even mentioned, let alone tested. exposed to a language to acquire it at all.
Other examples abound. Many scien- This compromise is, of course, not rea-
ti½c organizations have endorsed the sonable but false, as we see when chil-
slogan “violence is learned behavior,” dren exposed to a given language acquire
and even biologically oriented scientists it equally quickly regardless of their ra-
tend to treat violence as a public health cial ancestry. Though people may be ge-
problem like malnutrition or infectious netically predisposed to learn language,
disease. Unmentioned is the possibility they are not genetically predisposed,
that the strategic use of violence could even in part, to learn a particular lan-
have been selected for in human evolu- guage; the explanation for why people in
tion, as it has been in the evolution of different countries speak differently is
12other primate species. Gender differ- 100 percent environmental.
ences in the professions, such as that the Sometimes the opposite extreme turns
proportion of mechanical engineers who out to be correct. Psychiatrists common-
ly used to blame psychopathology on
10 Anne Fausto-Sterling, Myths of Gender: Bio- mothers. Autism was caused by ‘refrig-
logical Theories About Women and Men (New
erator mothers’ who did not emotionallyYork: Basic Books, 1985).
engage their children, schizophrenia by
11 David C. Rowe, The Limits of Family Influ- mothers who put their children in dou-
ence: Genes, Experience, and Behavior (New York: ble binds. Today we know that autism
Guilford Press, 1994); Judith Rich Harris, The
Nurture Assumption: Why Children Turn Out the 13 David Lubinski and Camilla Benbow, “Gen-
Way They Do (New York: Free Press, 1998). der Differences in Abilities and Preferences
Among the Gifted: Implications for the Math-
12 Martin Daly and Margo Wilson, Homicide Science Pipeline,” Current Directions in Psycho-
(New York: A. de Gruyter, 1988). logical Science 1 (1992) : 61–66.
Dædalus Fall 2004 5Pinker.qxd 09/29/04 2:54 PM Page 6
Steven and schizophrenia are highly heritable, chology, the possibility that heredity has
Pinker and though they are not completely de- any explanatory role at all is still inflam-on
termined by genes, the other plausible matory. human
nature contributors (such as toxins, pathogens,
and developmental accidents) have T he effects of genes depend crucially“
nothing to do with how parents treat on the environment, so heredity imposes
their children. Mothers don’t deserve no constraints on behavior.” Two exam-
some of the blame if their children have ples are commonly used to illustrate the
these disorders, as a nature-nurture point: different strains of corn may grow
compromise would imply. They de- to different heights when equally irrigat-
serve none of it. ed, but a plant from the taller strain
might end up shorter if it is deprived ofIf people recognized that every aspect water; and children with phenylke-“
of behavior involves a combination tonuria (pku), an inherited disorder
of nature and nurture, the political dis- resulting in retardation, can end up nor-
putes would evaporate.” Certainly mal if given a diet low in the amino acid
many psychologists strive for an in- phenylalanine.
nocuous middle ground. Consider this There is an aspect of this statement
quotation: that indeed is worth stressing. Genes do
not determine behavior like the roll of a
If the reader is now convinced that either
player piano. Environmental interven-
the genetic or environmental explanation
tions–from education and psychothera-
has won out to the exclusion of the other,
py to historical changes in attitudes and
we have not done a suf½ciently good job of
political systems–can signi½cantly af-
presenting one side or the other. It seems
fect human affairs. Also worth stressing
highly likely to us that both genes and en-
is that genes and environments may in-
vironment have something to do with this
teract in the statistician’s sense, namely,
issue.
that the effects of one can be exposed,
This appears to be a reasonable interac- multiplied, or reversed by the effects of
tionist compromise that could not pos- the other, rather than merely summed
sibly incite controversy. But in fact it with them. Two recent studies have
comes from one of the most incendiary identi½ed single genes that are respec-
books of the 1990s, Herrnstein and tively associated with violence and de-
Murray’s The Bell Curve. In this passage, pression, but have also shown that their
Herrnstein and Murray summed up their effects are manifested only with particu-
14argument that the difference in average lar histories of stressful experience.
iq scores between American blacks and At the same time, it is misleading to
American whites has both genetic and invoke environment dependence to deny
environmental causes. A “some-of-
14 Avshalom Caspi, Karen Sugden, Terrie E.
each” position did not protect them Mof½tt, Alan Taylor, and Ian W. Craig, “Influ-
from accusations of racism and compar- ence of Life Stress on Depression: Moderation
isons to Nazis. Nor, of course, did it by a Polymorphism in the 5-htt Gene,” Science
(2003): 386–389; Avshalom Caspi, Josephestablish their position was correct: as
McClay, Terrie E. Mof½tt, Jonathan Mill, Judywith the language a person speaks, the
Martin, and Ian W. Craig, “Evidence that the
black-white average iq gap could be 100
Cycle of Violence in Maltreated Children De-
percent environmental. The point is that pends on Genotype,” Science 297 (2002): 727–
in this and many other domains of psy- 742.
6 Dædalus Fall 2004Pinker.qxd 09/29/04 2:54 PM Page 7
Why nature the importance of understanding the matoes or pinecones. Chinese foot-bind-
& nurtureeffects of genes. To begin with, it is sim- ing is an environmental manipulation
won’t go
ply not true that any gene can have any that can radically affect the shape of the away
effect in some environment, with the foot, but it would be misleading to deny
implication that we can always design an that the anatomy of the human foot is
environment to produce whatever out- in an important sense speci½ed by the
come we value. Though some genetic genes, or to attribute it in equal parts to
effects may be nulli½ed in certain envi- heredity and environment. The point is
ronments, not all of them are: studies not merely rhetorical. The fact that kit-
that measure both genetic and environ- tens’ visual systems show abnormalities
mental similarity (such as adoption when their eyelids are sewn shut in a
designs, where correlations with adop- critical period of development does not
tive and biological parents can be com- imply (as was believed in the 1990s) that
pared) show numerous main effects of playing Mozart to babies or hanging col-
personality, intelligence, and behavior orful mobiles in their cribs will increase
16across a range of environmental varia- their intelligence.
tion. This is true even for the poster In short, the existence of environmen-
child of environmental mitigation, pku. tal mitigations doesn’t make the effects
Though a low-phenylalanine diet does of the genes inconsequential. On the
prevent severe mental retardation, it contrary, the genes specify what kinds
does not, as is ubiquitously claimed, ren- of environmental manipulations will
der the person ‘perfectly normal.’ pku have what kinds of effects and with what
children have mean iqs in the 80s and costs. This is true at every level, from the
90s and are impaired in tasks that expression of the genes themselves (as
depend on the prefrontal region of the I will discuss below) to large-scale at-
15cerebral cortex. tempts at social change. The totalitarian
Also, the mere existence of some envi- Marxist states of the twentieth century
ronment that can reverse the expected often succeeded at modifying behavior,
effects of genes is almost meaningless. but at the cost of massive coercion, ow-
Just because extreme environments can ing in part to mistaken assumptions
disrupt a trait does not mean that the about how easily human motives
ordinary range of environments will would respond to changed circum-
17modulate that trait, nor does it mean stances.
that the environment can explain the Conversely, many kinds of genuine
nature of the trait. Though unirrigated social progress succeeded by engaging
corn plants may shrivel, they won’t grow speci½c aspects of human nature. Peter
arbitrarily high when given ever-increas- Singer observes that normal humans in
ing amounts of water. Nor does their
dependence on water explain why they
16 John T. Bruer, The Myth of the First Three
bear ears of corn as opposed to to- Years: A New Understanding of Early Brain Devel-
opment and Lifelong Learning (New York: Free
15 Adele Diamond, “A Model System for Study- Press, 1999).
ing the Role of Dopamine in the Prefrontal Cor-
tex During Early Development in Humans: Ear- 17 Jonathan Glover, Humanity: A Moral His-
ly and Continuously Treated Phenylketonuria,” tory of the Twentieth Century (London: J. Cape,
in Handbook of Developmental Cognitive Neuro- 1999); Peter Singer, A Darwinian Left: Politics,
science, ed. Charles A. Nelson and Monica Evolution, and Cooperation (London: Weidenfeld
Luciana (Cambridge, Mass.: mit Press, 2001). & Nicolson, 1999).
Dædalus Fall 2004 7Pinker.qxd 09/29/04 2:54 PM Page 8
Steven all societies manifest a sense of sympa- temperature, hormones, the molecular
Pinker 21thy: an ability to treat the interests of environment, and neural activity.on 18human others as comparable to their own. Among the environmentally sensitive
nature Unfortunately, the size of the moral cir- gene-expression effects are those that
cle in which sympathy is extended is a make learning itself possible. Skills and
free parameter. By default, people sym- memories are stored as physical changes
pathize only with members of their own at the synapse, and these changes re-
family, clan, or village, and treat anyone quire the expression of genes in response
outside this circle as less than human. to patterns of neural activity.
But under certain circumstances the cir- These causal chains do not, however,
cle can expand to other clans, tribes, render the nature-nurture distinction
races, or even species. An important way obsolete. What they do is force us to
to understand moral progress, then, is to rethink the casual equation of ‘nature’
specify the triggers that prompt people with genes and of ‘nurture’ with every-
to expand or contract their moral circles. thing beyond the genes. Biologists have
It has been argued that the circle may be noted that the word ‘gene’ accumulated
expanded to include people to whom several meanings during the twentieth
22one is bound by networks of reciprocal century. These include a unit of hered-
19trade and interdependence, and that ity, a speci½cation of a part, a cause of a
it may be contracted to exclude people disease, a template for protein synthesis,
who are seen in degrading circum- a trigger of development, and a target of
20stances. In each case, an understand- natural selection.
ing of nonobvious aspects of human na- It is misleading, then, to equate the
ture reveals possible levers for humane prescienti½c concept of human nature
social change. with ‘the genes’ and leave it at that,
with the implication that environment-
dependent gene activity proves that hu-Genes are affected by their environ-“
man nature is inde½nitely modi½able byments, and learning requires the expres-
experience. Human nature is related tosion of genes, so the nature-nurture dis-
genes in terms of units of heredity, de-tinction is meaningless.” It is, of course,
velopment, and evolution, particularlyin the very nature of genes that they are
those units that exert a systematic andnot turned on all the time but are ex-
lasting effect on the wiring and chem-pressed and regulated by a variety of sig-
istry of the brain. This is distinct fromnals. These signals in turn may be trig-
the most common use of the term ‘gene’gered by a variety of inputs, including
in molecular biology, namely, in refer-
ence to stretches ofdna that code for a
18 Peter Singer, The Expanding Circle: Ethics
and Sociobiology (New York: Farrar, Straus & 21 Marcus, The Birth of the Mind; Ridley, Nature
Giroux, 1981). Via Nurture.
19 Robert Wright, NonZero: The Logic of Human 22 Ridley, Nature Via Nurture; Richard Dawk-
Destiny (New York: Pantheon Books, 2000). ins, The Extended Phenotype: The Gene as the Unit
of Selection (San Francisco: W. H. Freeman &
20 Glover, Humanity; Philip G. Zimbardo, Company, 1982); Seymour Benzer, “The Ele-
Christina Maslach, and Craig Haney, in Obedi- mentary Units of Heredity,” in A Symposium on
ence to Authority: Current Perspectives on the Mil- the Chemical Basis of Heredity, ed. William D.
gram Paradigm, ed. Thomas Blass (Mahwah, McElroy and Bentley Glass (Baltimore: Johns
N.J.: Lawrence Erlbaum Associates, 2000). Hopkins Press, 1957).
8 Dædalus Fall 2004Pinker.qxd 09/29/04 2:54 PM Page 9
Why nature protein. Some aspects of human nature perceptual environment are instructive
& nurturemay be speci½ed in information carriers in the sense that their effects are pre-
won’t go
other than protein templates, including dictable by the information contained in away
the cytoplasm, noncoding regions of the the input. Given a child who is equipped
genome that affect gene expression, to learn words in the ½rst place, the con-
properties of genes other than their se- tent of her vocabulary is predictable
quence (such as how they are imprint- from the words spoken to her. Given an
ed), and cross-generationally consistent adult equipped to understand contin-
aspects of the maternal environment gencies, the spot where he will park his
that the genome has been shaped by car will depend on where the No Parking
natural selection to expect. Conversely, signs are posted. But other aspects of the
many genes direct the synthesis of pro- environment, namely, those that affect
teins necessary for everyday metabolic the genes directly rather than affecting
function (such as wound repair, diges- the brain through the senses, trigger ge-
tion, and memory formation) without netically speci½ed if-then contingencies
embodying the traditional notion of that do not preserve information in the
human nature. trigger itself. Such contingencies are per-
The various concepts of ‘environ- vasive in biological development, where
ment,’ too, have to be re½ned. In most many genes produce transcription fac-
nature-nurture debates, ‘environment’ tors and other molecules that set off cas-
refers in practice to aspects of the world cades of expression of other genes. A
that make up the perceptual input to the good example is the Pax6 gene, which
person and over which other humans produces a protein that triggers the ex-
have some control. This encompasses, pression of twenty-½ve hundred other
for example, parental rewards and pun- genes, resulting in the formation of the
ishments, early enrichment, role mod- eye. Highly speci½c genetic responses
els, education, laws, peer influence, cul- can also occur when the organism inter-
ture, and social attitudes. It is misleading acts with its social environment, as
to blur ‘environment’ in the sense of the when a change of social status in a male
psychologically salient environment of cichlid ½sh triggers the expression of
the person with ‘environment’ in the more than ½fty genes, which in turn al-
sense of the chemical milieu of a chro- ter its size, aggressiveness, and stress
23mosome or cell, especially when that response. These are reminders both
milieu itself consists of the products of that innate organization cannot be
other genes and thus corresponds more equated with a lack of sensitivity to the
closely to the traditional notion of he- environment, and that responses to the
redity. There are still other senses of environment are often not speci½ed by
‘environment,’ such as nutrition and the stimulus but by the nature of the
environmental toxins; the point is not organism.
that one sense is primary, but that one
should seek to distinguish each sense Framing problems in terms of nature“
and characterize its effects precisely. and nurture prevents us from under-
A ½nal reason that the environment standing human development and mak-
dependence of the genes does not vitiate
23 Russell Fernald, “How Does Behavior
the concept of human nature is that an
Change the Brain? Multiple Methods to An-
environment can affect the organism in swer Old Questions,” Integrative Comparative
very different ways. Some aspects of the Biology 43 (2003): 771–779.
Dædalus Fall 2004 9Pinker.qxd 09/29/04 2:54 PM Page 10
Steven ing new discoveries.” On the contrary, but only half their variable genes); and
Pinker some of the most provocative discover- that biological siblings (who share theiron
ies in twentieth-century psychology environment and half their variablehuman
nature would have been impossible if there genes) are more similar than adoptive
had not been a concerted effort to dis- siblings (who share their environment
tinguish nature and nurture in human but none of their variable genes). These
development. studies have been replicated in large
For many decades psychologists have samples from several countries, and have
looked for the causes of individual dif- ruled out the most common alternative
ferences in cognitive ability (as mea- explanations (such as selective place-
sured by iq tests, school and job per- ment of identical twins in similar adop-
formance, and indices of brain activity) tive homes). Of course, concrete behav-
and in personality (as measured by ques- ioral traits that patently depend on con-
tionnaires, ratings, psychiatric evalua- tent provided by the home or culture–
tions, and tallies of behavior such as di- which language one speaks, which reli-
vorce and crime). The conventional gion one practices, which political party
wisdom has been that such traits are one supports–are not heritable at all.
strongly influenced by parenting prac- But traits that reflect the underlying tal-
tices and role models. But recall that this ents and temperaments–how pro½cient
belief is based on flawed correlational with language a person is, how religious,
studies that compare parents and chil- how liberal or conservative–are partially
dren but forget to control for genetic heritable. So genes play a role in making
relatedness. people different from their neighbors,
Behavioral geneticists have remedied and their environments play an equally
those flaws with studies of twins and important role.
adoptees, and have discovered that in At this point it is tempting to con-
fact virtually all behavioral traits are clude that people are shaped both by
partly (though never completely) herita- genes and by family upbringing: how
24ble. That is, some of the variation their parents treated them and what
among individual people within a cul- kind of home they grew up in. But the
ture must be attributed to differences in conclusion is unwarranted. Behavioral
their genes. The conclusion follows from genetics allows one to distinguish two
repeated discoveries that identical twins very different ways in which people’s
reared apart (who share their genes but environments might affect them. The
not their family environment) are highly shared environment is what impinges
similar; that ordinary identical twins on a person and his or her siblings alike:
(who share their environment and all their parents, home life, and neighbor-
their genes) are more similar than frater- hood. The unique environment is every-
nal twins (who share their environment thing else: anything that happens to a
person that does not necessarily happen24 Plomin, Owen, and McGuf½n, “The Genet-
to that person’s siblings. ic Basis of Complex Human Behaviors”; Eric
Turkheimer, “Three Laws of Behavior Genetics Remarkably, most studies of intelli-
and What They Mean,” Current Directions in gence, personality, and behavior turn up
Psychological Science 9 (5) (2000): 160–164; few or no effects of the shared environ-
Thomas J. Bouchard, Jr., “Genetic and Environ- ment–often to the surprise of the re-
mental Influences on Intelligence and Special
searchers themselves, who thought itMental Abilities,” Human Biology 70 (1998):
was obvious that nongenetic variation257–259.
10 Dædalus Fall 2004