NATURE VIA NURTURE - genes, experience, and what makes us human
http://www.amazon.com/exec/obidos/tg/detail/-/006000679X/qid=1093711993/

- INTELLIGENCE

Despite the sweeping successes of twin studies, a few features of
human behavior prove to be less heritable. The sense of humor shows
low heritability: adopted siblings seem to have quite similar senses
of humor, while separated twins have rather different ones. People's
food preferences seem to be barely heritable-you get your food
preferences from your early experience, not your genes (so do rats).
Social and political attitudes show a strong influence from the shared
environment-liberal or conservative parents seem to be able to pass on
their preferences to their children. Religious affiliation, too, is
passed on culturally, rather than genetically, though not religious
fervor.

What about intelligence? The debate about the heritability of IQ has
been scarred by controversy since its inception. The first IQ tests
were crude and culturally biased. In the 1920s, convinced that
intelligence was largely hereditary and alarmed at the thought of
excessive breeding by stupid people, governments in the United States
and many European countries began to sterilize mental defectives to
prevent them from passing on their genes. But in the 1960s came a
sudden revolution, as in so many other debates. From then on, even the
assertion of heritable IQ led to vitriolic campaigns of denunciation,
assaults on your reputation and demands for your dismissal. The first
to suffer this treatment was Arthur Jensen in 1969, following his
article in the Harvard Educational Review. By the 1990s, the argument
that society was segregating itself by assortative mating along
intellectual and therefore racial lines-asserted in The Bell Curve by
Richard Herrnstein and Charles Murray-provoked another wave of rage
among academics and journalists.

Yet I suspect that if you took a poll of ordinary people, they would
hardly have changed their views over a century. Most people believe in
"intelligence"-a natural aptitude or lack of it for intellectual
pursuits. The more children they have, the more they believe in it.
This does not stop them from also believing in coaxing it out of the
gifted and coaching it into the ungifted through education. But they
think that there is something innate.

The studies of twins reared apart or together unambiguously support
the idea that although some people are good at some things and others
are good at other things, there is such a thing as unitary
intelligence. That is to say, most measures of intelligence correlate
with each other. People who are good at general knowledge tests or
vocabulary tests are usually good at abstract reasoning or at tasks
that involve completing number series. This was first noticed a
century ago by a follower of Galton's, the statistician Charles
Spearman, who dubbed the common factor g for general intelligence.
Today, a measure of g derived from correlating different IQ tests
remains a powerful predictor of how well a child will do at school.
There has been more research on g than on any other subject in
psychology. Theories of multiple intelligence come and go, but the
notion of correlated intelligence just will not go away.

What is g? Something that appears so real in statistical tests must
surely have a physical manifestation in the brain. Is it something to
do with speed of thought or size of brain, or is it something subtler?
The first thing to be said is that the search for the genes of g has
been a huge disappointment. None of the genes that are capable of
causing mental retardation when broken prove to have any effect on
intelligence when altered more subtly. Searching at random through the
genes of intelligent people to find ways in which they consistently
differ from genes of normal people has so far turned up just one
decent statistical correlation (for the IGF2R gene on chromosome 6)
and more than 2,000 no-shows. This may just mean that the haystack is
too big and the needles too small. Candidate genes, such as the PLP
gene that seems to affect speed of neuronal transmission, have proved
capable of explaining only a small degree of reaction time and do not
correlate well with g: the speedy-brain theory of intelligence does
not look promising.

The one physical feature that does clearly predict intelligence is
brain size. The correlation between brain volume and IQ is about 40
percent, a number that leaves much room for the small-brained genius
and the big-brained dullard but is still a strong correlation. Brains
are composed of white matter and gray matter. When, in 2001, brain
scanners reached the stage that people could be compared for the
amount of gray matter in their brains, two separate studies in Holland
and Finland found a high correlation between g and volume of gray
matter, especially in certain parts of the brain. Both also found a
huge correlation between identical twins in volume of gray matter: 95
percent. Fraternal twins had only a 50 percent correlation. These
figures indicate something that is under almost pure genetic control,
leaving very little room for environmental influence. Gray matter
volume must be "due completely to genetic factors and not to
environmental factors" in the words of Danielle Posthuma, the Dutch
researcher. These studies bring us no closer to the actual genes of
intelligence, but they leave little doubt that the genes are there.
Gray matter consists of the bodies of neurons, and the new correlation
implies that clever people may literally have more neurons, or more
connections between neurons, than normal people do. After the
discovery of the role of the ASPM gene in determining brain size
through neuron number (chapter 1), it is beginning to look as if some
of the genes of g will soon be found.

However, g is not everything. Twin studies of intelligence also reveal
a role for the environment. Unlike personality, intelligence does seem
to receive a strong influence from the family. Studies of the
heritability of IQ in twins, adoptees, and combinations of the two
have all gradually converged on the same conclusion. IQ is
approximately 50 percent "additively genetic"; 25 percent is
influenced by the shared environment; and 25 percent influenced by
environmental factors unique to the individual. Intelligence therefore
stands out from personality in being much more susceptible to family
influence. Living in an intellectual home does make you more likely to
become an intellectual.

However, these average figures conceal two very much more interesting
features. First, you can find samples of people in which variation in
IQ is much more environmental and much less genetic than the average.
Eric Turkheimer found that the heritability of IQ depends strongly on
socioeconomic status. In a sample of 3 50 pairs of twins, many of whom
had been raised in extreme poverty, there emerged a clear difference
between the richest and the poorest. Among the poorest children
practically all the variability between individual IQ scores was
accounted for by shared environment and none by genetic type; in the
richer families, the opposite was true. In other words, living on a
few thousand dollars a year can severely affect your intelligence for
the worse. But living on $40,000 a year or $400,000 a year makes
little difference.

This is a finding with obvious significance for policy. It implies
that raising the safety net of the poorest does more to equalize
opportunity than reducing inequality in the middle classes. It is
dramatic confirmation of the truth I alluded to earlier: that even
when variation in achievement is explained entirely by genes, this
does not mean the environment does not matter. The reason you find
such strong genetic effects in most samples is that most of the people
in the samples live in adequately happy, supportive, affluent
families. If they did not, they would suffer enormously. It is a point
that is almost certainly true of personality, too. Your parents may
not have been able to alter your adult personality by being a little
bit strict. But you can be sure that they would have done so if they
had locked you in your room 10 hours a day for weeks on end.

Recall the heritability of weight. In a western society, with ample
access to food, those who put on weight faster will be the ones with
the genes that nudge them into eating more. But in a desolate part of
the Sudan, say, or Burma, where extreme poverty is rife and famine
just around the corner for many people, everybody is hungry and the
fat people are probably the rich ones. Here variation in weight is
caused by the environment, not the genes. In the jargon of the
scientist, the effect of the environment is nonlinear: at the
extremes, it has drastic effects. But in the moderate middle, a small
change in the environment has a negligible effect.

The second surprise hidden in the average figures is that the |
influence of genes increases and the influence of shared environment
gradually disappears with age. The older you grow, the less your
family background predicts your IQ and the better your genes predict
it. An orphan of brilliant parents adopted into a family of dullards
might do poorly at school but by middle age could end up a brilliant
professor of quantum mechanics. An orphan of dullard parents, reared
in a family of Nobel Prize-winners, might do well at school but by
middle age may be working in a job that requires little reading or
little deep thought.

Numerically, the contribution of "shared environment" to variation in
IQ in a western society is roughly 40 percent in people younger than
20. It then falls rapidly to zero in older age groups. Conversely, the
contribution of genes to explaining variation in IQ rises from 20
percent in infancy to 40 percent in childhood to 60 percent in adults
and maybe even 80 percent in people past middle age. In other words,
the effect of being reared in the same environment as somebody else is
influential while you are still in that environment but does not
endure beyond the period of shared rearing. Adoptive siblings do have
partly similar IQs while living together. But as adults their IQs are
wholly uncorrelated. By adulthood, intelligence is like personality:
mostly inherited, partly influenced by factors unique to the
individual, and very little affected by the family you grew up in.
This is a counterintuitive discovery exploding the old idea that genes
come early and nurture late.

What this seems to reflect is that the intellectual experience of a
child is generated by others. An adult, by contrast, generates his or
her own intellectual challenges. The "environment" is not some real,
inflexible thing: it is a unique set of influences actively chosen by
the actor himself or herself. Having a certain set of genes
predisposes a person to experience a certain environment. Having
"athletic" genes makes you want to practice a sport; having
"intellectual" genes makes you seek out intellectual activities. The
genes are agents of nurture.

As a parallel, how do genes affect weight? Presumably through
controlling appetite. In an affluent society, those who gain most
weight are hungrier and so eat more. The difference between a
genetically fat and a genetically thin westerner lies in the fact that
the first is more likely to buy ice cream. Is it the gene or the ice
cream that causes fatness? Well, it is obviously both. The genes are
causing the individual to go out and expose himself to an
environmental factor, in this case ice cream. Surely it is bound to be
the same in the case of intelligence. The genes are likely to be
affecting appetite more than aptitude. They do not make you
intelligent; they make you more likely to enjoy learning. Because you
enjoy it, you spend more time doing it and you grow more clever.
Nature can act only via nurture. It can act only by nudging people to
seek out the environmental influences that will satisfy their
appetites. The environment acts as a multiplier of small genetic
differences, pushing athletic children toward the sports that reward
them and pushing bright children toward the books that reward them.

The main conclusion in behavior genetics is counterintuitive in the
extreme. It tells you that nature plays a role in determining
personality, intelligence, and health-that genes matter. But it does
not tell you that this role is at the expense of nurture. If anything,
it proves rather dramatically that nurture matters just as much,
though it is inevitably less good at discerning how (there is no
environmental equivalent to the natural experiment created by
identical and fraternal twins). Gallon was utterly wrong in one
important respect. Nature does not prevail over nurture; they do not
compete; they are not rivals; it is not nature versus nurture at all.

Paradoxically, if western society has reached the point where the
heritability of intelligence is so high, then it means we have
achieved something approaching a meritocracy, where your background
does not matter. But this also reveals something truly surprising
about genes. They do vary within the normal range of human behavior.
You might expect that genes would be like vitamin C or families-they
become limiting only when they are malfunctional. So broken genes
might cause rare broken minds, just as they cause rare diseases.
Severe depression, mental illness, or mental disability might be
caused by rare variations in genes, just as all these things could be
caused by a rare and bizarre upbringing. This would then be the
perfect Utopia in which, so long as all had normal genes and a normal
family, everybody would have the same potential personality and
intelligence. The details would then come down to accident or
circumstance.

But it is not like that. Behavior genetics reveals very starkly that
there are genetic differences which are common and which affect our
personalities within the range of normal human experience. There are
val-vals and met-mets among us, not just for the BDNF gene but for
many other genes affecting personality, intelligence, and other
aspects of the mind. Just as some people are genetically better at
gaining muscle strength than others, according to which version of the
ACE gene they possess on chromosome 17, so some people are genetically
more able to absorb education according to which versions they possess
of some unknown genes. These mutations are not rare; they are common.

From the point of view of the evolutionary biologist this is a
scandal. Why is there so much "normal" genetic variation, or, to give
it its proper name, polymorphism? Surely, the "clever" variants on
genes would gradually drive the "dull" ones to extinction, and the
phlegmatic ones would drive out the excitable ones. One kind must
inevitably be superior to the other in providing survival or mating
advantages. One kind must therefore endow its owner with greater
ability to become a fecund ancestor. Yet there is no evidence of genes
going extinct in this way. There seems to be a sort of happy
coexistence of different versions of genes within the human
population.

Enigmatically, there is more genetic variation in the human population
than science has a right to expect. Behavior genetics, remember, does
not discover what determines behavior; it discovers what varies. And
the answer is that genes vary. Contrary to popular opinion, most
scientists love enigmas. They are in the business of finding new
mysteries, not cataloging facts. The white-coated ones in the labs
live in the dim hope of finding a really fine conundrum or paradox.
And here is a fine one.

There are plenty of theories to explain the enigma, though none that
is entirely satisfactory. Perhaps we human beings have simply relaxed
natural selection so much by keeping ourselves alive with technology
that our mutations have proliferated. But then why is the same
variation present in other animals? Perhaps there is a delicate form
of balancing selection that always favors the rare variants, thus
keeping rare genes from going extinct. This idea certainly seems to
explain variability in the immune system because disease favors rare
versions of genes by attacking the common ones, but it is not
immediately obvious why this should preserve polymorphism in
personality. Perhaps mate choice encourages diversity. Or perhaps some
new idea, as yet unheard of, will explain the phenomenon. Rival
explanations for polymorphism were already causing bitter divisions
among evolutionists in the 1930s, and they are not settled yet.

NATURE VIA NURTURE - genes, experience, and what makes us human
http://www.amazon.com/exec/obidos/tg/detail/-/006000679X/qid=1093711993/

------------------------------------

...the features that we depend on to identify whether a person is
caucasoid, negroid, or mongoloid, etc., are the superficial soft parts
of the body. Lips, noses, hair, eyes, and skin do not fossil- ize. At
the same time, the hard parts that do get preserved are not reliable
as racial markers because almost all of the skeletal dimensions of all
the races overlap. But there is a more profound problem with trying to
say how long the contemporary races have been in existence. Genes that
determine features used for denning contemporary races need not form
permanently associated hereditary bundles of traits. Variants of skin
color, hair form, lip size, nose width, eye folds, and so on can be
assorted and inherited independently of each other. This means that
the traits that go together today did not necessarily go together in
the past, or indeed even existed in the past, among the populations
that were ancestral to today's racial groupings.

Even today, there are so many different combinations of racial traits
around the world that no simple scheme of four or five major racial
types can do justice to them. Millions of people with thin lips, thin
noses, and wavy hair, but dark brown to black skin, live in North
Africa. Native inhabitants of southern Africa, such as the San, have
epicanthic eye folds (like most Asians), light brown to dark brown
skin, and tightly spiraled hair. India has people with straight or
wavy hair, dark brown to black skin, and thin lips and thin noses. On
the steppes of central Asia, epicanthic eye folds combine with wavy
hair, light eyes, considerable body and facial hair, and pale skins.
Indonesians have a high frequency of epicanthic eye folds, light to
dark brown skin, wavy hair, thick noses, and thick lips. Inhabitants
of the islands of Oceania present combinations of brown to black skin,
with contrastive forms and quantities of hair and facial features. An
interesting bundle of traits occurs among the Ainu of northern Japan,
who have light skin and thick browridges, and are the hairiest people
in the world. In Australia, pale to dark brown skin color and wavy
blond to brown hair are common.

Ignorance or denial of the separability of traits used for racial
identity can lead people to create strange biological categories. The
distinction between blacks and whites in the United States, for
example, ignores the obvious fact that individual blacks can have
eyes, nose, hair, and lips that are indistinguishable from these
features among whites. The reverse is also true of whites, among whom
some individuals look more negroid than some blacks. These anomalies
occur because Americans do not mean by race what people actually look
like as determined by their genes but by how their parents were
classified. According to this conception of race, if one parent is
"black" and the other is "white," their child is "black," despite the
fact that by the laws of genetics, half of a child's genes are from
the black parent and half from the white. The practice of cramming
people into these racial pigeonholes becomes absurd when black
ancestry consists of only a single grandparent or great-grand-parent.
This produces the phenomenon of the white who is socially classified
as "black." Most American blacks have received a significant portion
of their genes from recent European ancestors. When samples of
American blacks are studied, the assumption that they genetically
represent Africans is incorrect. Perhaps we would do well to emulate
the Brazilians, who identify racial types not by three or four terms
but by 300 to 400, in proper deference to the fact that people whose
parents and grandparents were a mixture of Europeans, Africans, and
American Indians cannot be said to be either Europeans, Africans, or
American Indians.

Traits that we can see don't stick together with those we can't. Take
the ABO blood groups. Between 70 percent and 80 percent of light-
skinned Scots, black-skinned central Africans, and brown-skinned
Aborigines of Australia all have type 0. If we could see type 0 blood
groups the way we see skin color, would we put the Scots and the
Africans in the same race? Type A is equally unmindful of skin-deep
distinctions. Africans, East Indians, and Chinese all have 10 percent
to 20 percent frequencies of type A. Should we put them all in the
same race?

Another example of an invisible trait that blithely ignores
conventional racial boundaries is the ability to taste PTC
(phenylrhiocarbamide). In 1931, a laboratory researcher accidentally
dropped a sample of this substance. Fellow workers complained about
the bitter taste that it produced in their mouths; others said they
tasted nothing. Anthropologists now know that the world is divided
into PTC-tasrers and non-PTC-tasters. In Asia, nontasters range from
15 percent to 40 percent. There are twice as many nontasters in Japan
as in China, and three times as many in Malaysia. Does this mean that
each of these groups belongs to a separate race? If tasters could see
nontasters, would they make fun of them and refuse to let them into
their neighborhoods or their schools?

New combinations and frequencies of genes have kept the species'
racial types in a state of flux ever since populations of modern
sapiens began to spread throughout Africa and Eurasia. Some of these
changes reflect the workings of chance. During migrations by small
groups into new regions, the settlers by accident may happen to have
had a high frequency for a gene that was rare in their ancestral
population. Thenceforth, the new population had a high frequency of
the variant. Such a scenario could account for the distinctive shovel
shape of the incisors of Asian peoples.

An accelerated flow of genes when migrants encounter genetically
distinct populations is another essentially random process
contributing to the evanescence of racial types. During earlier times,
nothing quite so massive as the blending of races in the United States
and Brazil could have happened, yet some degree of race mixture would
have been unavoidable at the shifting boundaries between genetically
distinct populations in remotest antiquity.

Finally, as is generally true of all biological evolution, a major
cause of the shifting distribution and frequency of genes
conventionally used to identify racial divisions is natural selection.
As populations move into different habitats or as environments change,
selection for reproductive success leads to the appearance of new
bundles of hereditary traits.

Anthropologists have made a number of plausible suggestions relating
racial differences to temperature, humidity, and other climatological
factors. For example, long, narrow noses of Europeans may have been
selected to warm extremely cold, damp air to body temperature before
it reached the lungs. The generally rounded, squat bodies of Eskimo
may also represent adaptation to cold-Bergman's rule again. A tall,
thin body, in contrast, leads to maximum heat loss. And this may
explain the tall, thin bodies of Nilotic Africans, who inhabit regions
of intense arid heat and whose descendants make some of the world's
greatest basketball players.

Ironically, traits whose frequencies are determined by natural
selection are not very good markers for the purpose of reconstructing
the history and antiquity of today's racial divisions. Suppose, for
example, that people with short noses migrate from a tropical to a
cold climate. Within a few-score generations, natural selection will
increase the frequency of long noses'among them. An observer noting
the similarity between them and their long-nosed neighbors might
readily conclude that they were descended from a long-nosed cold-
climate race rather than a short-nosed hot-climate one. So the best
markers of racial ancestry are traits that are accidental or
nonadaptive, like the shovel-shaped incisors I mentioned a moment
ago.

Unfortunately, many of the traits that anthropologists once thought
were the best markers of racial ancestry have turned out to have
adaptive value in certain contexts. Blood groups were a particularly
keen disappointment, for it turns out that the ABO series is linked to
resistance to diseases that may affect reproductive success, such as
smallpox, bubonic plague, and food poisoning by toxic bacteria. So the
explanations for blood-type frequencies probably lie as much in the
history of transient exposures of different populations to different
diseases as in racial ancestry. Even a trait as cryptic and seemingly
useless as the ability to taste PTC may not indicate common descent as
much as similar adaptive responses by ancestrally separate
populations. Chemically, PTC resembles certain substances that have
adverse effects on the functioning of the thyroid gland. A common
consequence of thyroid malfunction is goiter, a crippling, life-
shortening disease. In populations at risk for goiter, the ability to
taste foods containing the PTC-like thyroid-inhibiting substances
would be selected for, rendering the distinction between taster and
nontaster unreliable for reconstructing racial ancestry.

OUR KIND by Marvin Harris 1989
http://www.amazon.com/exec/obidos/tg/detail/-/0060919906/

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