Comment by Paul Ehrlich &

Marcus W. Feldman

Genes, environments & behaviors

Our large brains are surely at the cen-

ter of our humanity. But it is equally cer-
tain that few organs are the subject of
more misinformation in scienti½c and

Paul Ehrlich, a Fellow of the American Academy
seit 1982, is Bing Professor of Population Stud-
ies and president of the Center for Conservation
Biology at Stanford University. He is the author
of numerous publications, including “The Pop-
ulation Bomb” (1968), “The End of Affluence”
(with Anne H. ehrlich, 1974), “Human Na-
tures: Genes, Culture, and the Human Prospect”
(2000), and “One with Nineveh: Politik, Con-
sumption, and the Human Future” (with Anne
H. ehrlich, 2004).

Marcus W. Feldman, a Fellow of the American
Academy since 1987, is Burnet C. and Mildred
Finley Wohlford Professor and director of the
Morrison Institute for Population and Resource
Studies at Stanford University. He has published
extensively in scienti½c journals such as “Science,”
“Nature,” and “Evolution.” His current research
interests include the evolution of complex genetic
systems that can undergo both natural selection
and recombination; human molecular evolution;
and the interaction of biological and cultural evo-
lution.

public discourse–especially in the wide-
spread notion that most behaviors con-
trolled by our marvelous brain are some-
how programmed into it genetically. A
typical treatment in the popular press
is this overexcited claim by columnist
Nicholas Wade in the New York Times:
“When . . . [the human genome] . . . is ful-
ly translated, it will prove the ultimate
thriller–the indisputable guide to the
graces and horrors of human nature, Die
creations and cruelties of the human
Geist, the unbearable light and darkness
of being.”1

Wade may get a pass for being a jour-

nalist, but some scientists are equally
confused. Molecular biologist Dean
Hamer wrote: “People are different be-
cause they have different genes that cre-
ated different brains that formed differ-
ent personalities,” and “[u]nderstand-
ing the genetic roots of personality will
help you ‘½nd yourself’ and relate better
to others.” As distinguished a neurobiol-
ogist as Michael Gazzaniga is guilty of

1 The authors thank Richard Lewontin, Deb-
orah Rogers, Robert Sapolsky, and Michael
Soulé for their comments on earlier versions of
the manuscript. N. Wade, “Ideas and Trends:
The Story of Us; The Other Secrets of the Ge-
nome,” New York Times, Februar 18, 2001, Sek.
4, 3.

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Kommentar
by Paul
ehrlich &
Marcus W.
Feldman

the misleading claim that “all behavior-
al traits are heritable”;2 and molecular
evolutionists Roderick Page and Edward
Holmes have asserted that “genes con-
trol 62% of our cognitive ability.”3 In
fact, an entire neo-½eld labeled evolu-
tionary psychology has sprung up based
on the misconception that genes are
somehow determining our everyday be-
havior and our personalities. It is a ½eld
that believes there are genetic evolution-
ary answers to such questions as why a
man driving an expensive car is more at-
tractive than one driving a cheap car.4

So even well-educated and thoughtful

observers have been persuaded by the
language of heritability. With expres-
sions such as ‘genes are responsible for
50 percent of,’ or ‘genes contribute 50
percent of,’ a behavior, this language
gives the impression that genetic and
environmental contributions to human
behaviors are actually separable. Sie
are not.

Heritability was originally introduced
in the 1930s in the context of agriculture.
It is an index of amenability to selective
breeding under environmental condi-
tions that the breeder could control.
This index, now often termed ‘narrow-
sense heritability,’ is the fraction of all
variation in a trait that can be ascribed
only to genes that act independently of
one another and whose joint effect is
the sum of their individual effects. Eins
easy-to-understand way of measuring
heritability is through a one-generation
selection experiment. Individuals with

2 M. S. Gazzaniga, The Ethical Brain (Neu
York: Dana Press, 2005), 44.

3 R. D. M. Page and E. C. Holmes, Molekular
Evolution: A Phylogenetic Approach (Oxford:
Blackwell, 1998), 119.

4 D. M. Buss, The Evolution of Human Desire
(New York: BasicBooks, 1994), 99–100.

extreme values of a trait are bred to one
another–for example, the heaviest indi-
viduals from a hog population. The off-
spring are then raised in the same envi-
ronment, and their average weight cal-
culated. If the average weight of the off-
spring doesn’t increase over that of the
entire population (not just of the heavy
Eltern) in the previous generation, Die
heritability is zero. Andererseits,
if the average weight of the offspring
equals that of their heavy parents, Die
heritability is 100 Prozent.

In den 1960ern, the term ‘heritability’
was adopted by some students of human
behaviors who wanted to know what
fraction of the variation in these behav-
iors was primarily attributable to genetic
differences and what percentage to envi-
ronmental differences. Because control-
ling the environments of human subjects
is not possible, Jedoch, this fraction–
now called ‘broad-sense heritability’–
includes variation from interactions be-
tween genes and environments. Das
fraction of variation is nevertheless in-
terpreted as determined by genes, daher
inflating the heritability.

Mit anderen Worten, this new heritability

statistic assumes no relationship be-
tween genetic transmission and envi-
ronment, z.B., that the iq scores of par-
ents cannot affect those parts of the
environment that might interact with
genes to influence a child’s iq. Der
amount of stimulation parents provide
their young children, the nature of din-
ner-table conversations, and the number
of books in the home are thus taken to
be independent of any genetic influences
on children’s iq. When this indepen-
dence assumption is violated, there is
gene-environment correlation–exactly
the correlation that agricultural experi-
ments to estimate narrow-sense heri-
tability eliminated by holding environ-
ments constant. But with human behav-

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Genes, In-
vironments
& behaviors

iors such designs are impossible, und das
correlation between parental iq and the
offspring’s environment may contribute
to the heritability.

Many of the high estimates for heri-
tability, and the resulting interpretation
that human behavioral traits are heavily
influenced by genes, have been derived
from comparisons of identical twins
(who originate from a single fertilized
egg) and fraternal twins (from two
Eier). These estimates are based on the
fact that identical twins share exactly
the same hereditary endowment, while
fraternal twins, on average, share only
50 percent of their genes.

But many assumptions about twins
inflate twin-based estimates of broad-
sense heritability. One is the ‘equal en-
vironments’ assumption, that variation
in environments created by parents to
which identical twin pairs are exposed
is the same as those to which fraternal
pairs are exposed–i.e., that there is no
difference between the way parents treat
identical and fraternal twins. Statistical
estimates of the differences in the envi-
ronmental exposure of identical and fra-
ternal twins outside of the parental contri-
bution, Jedoch, are not usually made.
Some studies have found that the corre-
lation between iq and the environments
not transmitted by the parents of identi-
cal twins is much higher than that of fra-
ternal twins.5 Thus, factors in the non-
familial environment of identical twins
are often more similar than those of fra-
ternal twins, but this difference between
identical and fraternal twins is usually
ignored.

5 C. R. Cloninger, J. Rice, and T. Reich, “Mul-
tifactorial Inheritance with Cultural Trans-
mission and Assortative Mating,” American
Journal of Human Genetics 31 (1979): 176–198;
M. W. Feldman and S. Otto, “Twin Studies,
Heritability and Intelligence,” Science 278
(1997): 1383–1384.

It might be thought that some of the
problems with twin studies may be over-
come if the identical twins under study
were reared apart, das ist, in different
Familien. In a perfect experiment of this
kind, all observed differences between
the twins should be environmental,
and high levels of similarity of the pair
should be due to their identical genes. Es
turns out not to be so simple. Erste, sepa-
rated twin pairs are rare, and the reasons
for the separation are not usually known.
Zweite, the twins share the prenatal en-
vironment of the ovary, fallopian tube,
and uterus, which could be very influen-
tial in producing similar developmental
pathways. Dritte, the separation is fre-
quently carried out well after birth so
some shared early postnatal environ-
mental effects could mistakenly be inter-
preted as genetic. Vierte, twins have of-
ten been placed in separate homes that
are similar in aspects that may be impor-
tant for the traits under study, for exam-
Bitte, in homes of relatives of their par-
ents. The environments are thus not a
random sample of all possible environ-
gen. Kamin and Goldberger docu-
mented these problems with the well-
publicized Minnesota study of twins
reared apart.6 All of these effects add to
that component of variation that is in-
terpreted as genetic, with the result that
estimates of genetic heritability based
on identical twins raised separately are
biased upward.

At ½rst glance, some of the stories of
the similarities of identical twins raised
separately seem extraordinary examples
of the power of genetic identity. Two
men separated near birth grow up to be
beer-drinking ½re½ghters and grasp the
beer cans in the same unusual way, hold-

6 L. J. Kamin and A. S. Goldberger, “Twin
Studies in Behavioral Research: A Skeptical
View,” Theoretical Population Biology 61 (2002):
83–95.

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Kommentar
by Paul
ehrlich &
Marcus W.
Feldman

ing the little ½nger under the can.7 But
they were raised in similar lower middle-
class Jewish homes in New Jersey. Sei-
ing a ½re½ghter is an ambition of many
Männer, and ½re½ghters are not notorious
for being addicted to wine. Außerdem,
it is well known that physical attributes
of people greatly influence how other
people treat them. Individuals with iden-
tical genomes are usually strikingly alike
in appearance, and within the same cul-
ture they will be treated more similarly
than randomly selected individuals of
the same gender from the same occu-
pational and age groups. Resemblance
in body structure (strong in identical
twins) would probably also make it com-
fortable to hold containers in the same
manner, and we doubt if even the most
dedicated hereditarian would seek a
gene for use of the pinky in beer drink-
ing.

Ever since narrow-sense heritability
was ½rst used, it has been well under-
stood by geneticists that an estimate of
the genetic influence on a trait’s variabil-
ity depends on the particular population
and the particular environment in which
the trait was measured. Außerdem,
even a very high heritability measured
in a population cannot be used to infer
something about any single member of
that population. Suppose a population
is known to have higher than average
blood pressure. Would a physician treat-
ing one individual patient from that pop-
ulation prescribe an antihypertensive
drug on the basis of the population sta-
tistic? Of course not–a doctor would
use detailed history and laboratory
workup to decide on the appropriate
treatment for that particular patient.
The patient’s diet or stress level (Die

7 N. L. Segal, Indivisible by Two: Lives of Extraor-
dinary Twins (Cambridge, Masse.: Harvard Uni-
versity Press, 2005).

Umfeld) would be critical to the
medical recommendation and, in most
Fälle, likely to overwhelm any genetic
effect inferred from population studies.
The logic of using the heritability of
some trait in a population to predict
something about a member of that pop-
ulation would be foolish.

Recent studies of intelligence in sam-
ples of twins of different socioeconom-
ic status strongly reinforce these restric-
tions on the generalization of heritabili-
ty. Zum Beispiel, the estimated heritabili-
ty of iq in individuals from advantaged
backgrounds is signi½cantly higher
than in those from disadvantaged back-
grounds.8 That is because better envi-
ronments allow more variance in iq to
be expressed: potential geniuses have
trouble developing into Einsteins in
slums without schools. Likewise, Die
heritability of height in a normal human
population would be greater than that in
a starved one, where everyone’s growth
is stunted and the variance in height
thereby reduced.

Individuals with Down syndrome,
caused by an entire additional chromo-
manche 21 (trisomy), develop as severely
mentally handicapped if given no spe-
cial treatment. But it turns out that the
degree of handicap is extremely labile
to the environment of rearing.9 In fact,
the day may come when an environment
can be provided in which their develop-
ment will be entirely normal. Moreov-
er, not even evolutionary psychologists

8 E. Turkheimer, A. Haley, M. Waldron, B.
D’Onofrio, and I. ICH. Gottesman, “Socioeconom-
ic Status Modi½es Heritability of iq in Young
Children,” Psychological Science 14 (2003): 623–
628.

9 R. ICH. Braun, “Down Syndrome and Quality
of Life: Some Challenges for Future Practice,”
Down Syndrome Research and Practice 2 (1994):
19–30; N. J. Roizen and D. Patterson, “Down’s
Syndrome,” The Lancet 361 (2003): 1281–1289.

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have proposed that chromosome 21 Ist
the locus of ‘the intelligence gene.’

Such important gene-environment
interactions preclude the partition of
variation in traits like trisomy, iq, oder
height into genetic and nongenetic in-
fluences. It is especially inappropriate
to talk about genetic ‘contributions’ to
such complex traits when in some envi-
ronments genetic variation is not even
detectable. It is equally incorrect to say,
‘characteristic A is more influenced by
nature than nurture,’ as it is to say, ‘the
area of a rectangle is more influenced
by its length than its width.’ (Beachten Sie, dass
the area of a rectangle one hundred
miles long and one inch wide is halved
by reducing its length by ½fty miles or
by reducing the width by half an inch.)
None of this should be taken to mean
that genes do not affect behavior. Tatsächlich,
in a sense, they influence all behavior, bei
least by laying out how human capabili-
ties differ from those of other primates.
If genes did not, in the course of devel-
opment, interact with pre- and postna-
tal environments to generate the brain–
some of the major patterns of its organi-
zation, and its principal modes of inter-
action with hormonal systems–the hu-
man behaviors that interest us would
not occur at all. Genomic disparities be-
tween species doubtless influence differ-
ences in the general con½guration of the
systems that control behavior.

But it is clear from the long pre- Und
(especially) postnatal environmental
programming that these systems must
undergo to produce a behaviorally ‘nor-
mal’ person that genes are not responsi-
ble for embedding detailed instructions
on how to act, or even ‘tendencies’ to-
ward certain kinds of behavior. Environ-
mental inputs are so extensive that the
cortex of the brain is not fully developed
until the mid-twenties. In view of this,
it’s not surprising that nothing indicates

that genes favored by selection while
our ancestors were hunter-gatherers
signi½cantly influence such contempo-
rary individual behavioral characteris-
tics as choice of beers or marriage part-
ners.

For many behavioral traits, especially
serious psychiatric disorders, some in-
dividual genes have been shown to play
a role in some environments but not in
Andere. Consider research by psycholo-
gist Avshalom Caspi and his colleagues
on the effects of having different forms
of a gene involved in the transport of
serotonin, a compound that is involved
in transmitting signals along certain
nerve pathways. Which form an indi-
vidual possesses apparently influences
whether stressful events will produce
depression. Having the ‘wrong’ gene,
Jedoch, only makes a difference if an
individual is exposed to a stressful envi-
ronment early in adult life–a beautiful
example of gene-environment interac-
tion.10

Many other cases illuminate the fail-
ure of genes to ‘control’ behavior. Der
original Siamese twins, Chang and Eng,
were joined for life by a narrow band of
tissue connecting their chests. Despite
their identical genomes, they had very
different personalities. One was an al-
coholic, the other sober; one was domi-
nant, the other submissive. Equally fas-
cinating is the story of the Dionne quin-
tuplets, ½ve genetically identical little
girls who, in the 1930s, were essentially
raised in a laboratory under the super-
vision of a psychologist. When the girls
were only ½ve, the psychologist wrote a
book that expressed his astonishment at
how different the little girls were–some-
thing con½rmed by their very different

10 M. Rutter, Genes and Behavior: Nature-Nur-
ture Interplay Explained (Oxford: Blackwell,
2005).

Genes, In-
vironments
& behaviors

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Dædalus Spring 2007

Kommentar
by Paul
ehrlich &
Marcus W.
Feldman

life trajectories. One had epilepsy, Die
others did not; some died young, Die
others old; some married, others re-
mained single; und so weiter. Ähnlich,
the identical Marks triplets grew up
with different sexual orientations, zwei
straight and one gay; one of the two
identical Perez girls chose to change
her sex with hormones and surgery and
married a woman, while the other twin
remained female and married a man.11
But one does not even have to look at
such extreme cases to see that genes are
not controlling human actions; evidence
that common behaviors are not geneti-
cally determined is superabundant. Per-
haps the most impressive comes from
thousands of cross-cultural ‘experi-
ments’ in which children from one cul-
ture are raised from an early age by
adoptive parents from another. Invari-
ably, the children mature with the lan-
guage and attitudes of the adoptive cul-
tur.

Also impressive is the ease with which

culture overrides the only ‘command-
ment’ we can be sure is contained in
everyone’s dna: ensure that your genes
are maximally represented in the next
Generation, either by having more chil-
dren or by helping your relatives (WHO
tend to have the same genes) to repro-
duce. Differential reproduction of genet-
ically different individuals (not explica-
ble by chance) is natural selection, Die
creative force in evolution. We wouldn’t
be here if our ancestors hadn’t been ef-
fective reproducers of their genes, if they
hadn’t had high ‘½tness.’ But culture
(part of the environment) has led human
beings to limit their reproduction as far
back in history as we can trace, all the
way to the ancient Egyptians who used
crocodile-dung suppositories as contra-

11 Segal, Indivisible by Two: Lives of Extraordi-
nary Twins.

ceptives (which we are convinced were
very effective!).12 In der Tat, although evo-
lutionary psychologists like to imagine
that rapists are programmed to assault
women in order to reproduce themselves
–that is, to increase their ½tness–over
half of all rapes occur in circumstances
(z.B., victims too old or too young, NEIN
ejaculation into the vagina) where fertil-
ization is impossible, and in more than
a ½fth of cases more force is used than
would be required to achieve the sup-
posed reproductive goal.13

Most de½nitive, obwohl, is the prob-

lem of gene shortage.14 Our roughly
twenty-½ve thousand genes can’t pos-
sibly code all of our separate everyday
behaviors into the human genome. Von-
Alles klar, we have less than twice as many
as required to make a fruit fly, und nur
a few more than those that lay out the
ground plan of a simple roundworm.
Even if the human brain had not evolved
for flexibility but instead were pro-
grammed for stereotypic behavior, unser
genes couldn’t store enough informa-
tion to accomplish it. Genes are not lit-
tle beads with instructions like ‘grow
up gay’ engraved on them. They are in-
structions that, in a very complex mech-
anism, can be translated into a sequence
of amino acid residues in a protein. Es
is near miraculous that these proteins–
interacting with each other, Funktion-

12 L. Manniche, Sexual Life in Ancient Egypt
(New York: Kegan Paul, 1997).

13 J. Coyne, “Of Vice and Men: A Case Study
in Evolutionary Psychology,” in Evolution, Gen-
der, and Rape, Hrsg. C. Travis (Cambridge, Masse.:
mit Press, 2003), 171–189.

14 P. R. ehrlich, Human Natures: Genes, Cul-
tures, and the Human Prospect (Washington,
D.C.: Island Press, 2000); P. R. Ehrlich and
M. W. Feldman, “Genes and Cultures: Was
Creates Our Behavioral Phenome?” Current
Anthropology 44 (2003): 87–107.

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ing in different physical, physiological,
and social environments, and helping
to control the production of other pro-
teins–are able to produce an entire hu-
man body and the basic scaffolding for
a brain with a trillion or so nerve cells
(Neuronen) connected to each other by
tens of trillions of intricate junctions
(synapses). On average, each gene must
influence many characteristics. Dort
are obviously enough genes, interacting
with each other and with diverse envi-
ronments at all scales, to provide a brain
that can generate all observed human
behaviors. But this has confused some
observers into thinking that because one
gene normally affects many functions
there is no gene shortage.

That fact is actually the basis of calling

it gene shortage. It means that natural
selection altering the genome to encode
one behavior would inevitably change
other aspects of the genome as well–so
that selection increasing, sagen, the speed
of contraction of muscle ½bers would
quite possibly modify the connections
between some neurons that, sagen, trans-
mit visual information from the retina
to the brain. Because of the small num-
ber of genes in the human genome and
the ubiquity of interactions between
proteins and between proteins and envi-
ronments, natural selection must ordi-
narily entrain a multiplicity of changes.
It must operate on a genome enormous-
ly ‘ampli½ed’ in development by the
multiple uses of the proteins produced
by single genes, by the alternative ways
the proteins are assembled, by the small
rna molecules that often control the
expression of multiple genes, and by the
epigenetic phenomena that may have
differing effects even on identical geno-
types.15

15 M. F. Fraga et al., “Epigenetic Differences
Arise During the Lifetime of Monozygotic

This may be why it has been so dif½-
cult to demonstrate that natural selec-
tion has changed more than a tiny frac-
tion of genes during the transition from
chimpanzee to modern human being.
Changing just a few genes can have ef-
fects that totally transform an entire or-
ganism. Daher, most population geneti-
cists–remembering linkage, pleiotropy,
epistasis, and developmental complexi-
ty–reject evolutionary psychology as a
theoretical paradigm: its predictions ig-
nore how dif½cult gene-gene and gene-
environment interactions make it for
selection to operate on just one pheno-
typic attribute. If we had trillions of
largely independent genes, then it might
be possible for selection (were it strong
enough and time available long enough)
to program us to rape, be honest, detect
cheaters, excel at calculus, or vote Re-
publican. But the number of independent
genes is much smaller than twenty-½ve
tausend.

Perhaps the most interesting thing
about all the attention paid to whether
nature or nurture controls behaviors is
not that individuals with identical ge-
nomes often behave very differently, Aber
that those same individuals exposed to
extremely similar environments also
turn out to behave quite differently. Das
has been clearly demonstrated in mice,
where genetically uniform strains ex-
posed to laboratory environments made
as identical as possible still behaved dif-
ferently.16 Indeed, nonidentical human
siblings, who share half of their genes,
the same parents, and apparently very
similar environments, often seem more

Twins,” Proceedings of the National Academy of
Sciences usa102 (2005): 10604–10609.

16 J. C. Crabbe, D. Wahlsten, and B. C. Von-
dek, “Genetics of Mouse Behavior: Interac-
tions with Laboratory Environment,” Science
284 (1999): 1670–1672.

Genes, In-
vironments
& behaviors

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Dædalus Spring 2007

siblings, including identical twins? Wir
hypothesize that there may be a ‘sibling
bifurcation’ phenomenon, in which in-
dividuals having close relationships with
others early in life, either pre- or post-
natal, often seek different life courses.
This could be related to such things as a
kin-recognition/inbreeding avoidance
System; attempts by parents, siblings,
Lehrer, and peers to distinguish relat-
ed individuals; genetic differences (Sei-
tween fraternal twins); birth-order ef-
fects; und so weiter.

We now know more than enough
about the human genome and human
development to see that the notion of
‘genes for behaviors’ is misguided. Für
complex traits such as normal behaviors,
few cases have been found where a spe-
ci½c gene, or even many genes, greatly
influences variation in the trait. Es ist
clear that when genes influence traits,
including behaviors, they only do so in
ways that are affected by environments.
Thus environments during any phase of
life might alter the way in which an indi-
vidual’s genes function in those environ-
gen. Das ist, Natürlich, a tribute to the
marvelous plasticity of the human brain,
which neurobiologists know changes in
response to external and internal envi-
ronments throughout life. It also makes
ridiculous the claim that genes program
our behaviors or, In der Tat, that genes are
responsible for some speci½ed fraction
of any human behavior.

Kommentar
by Paul
ehrlich &
Marcus W.
Feldman

unalike than unrelated people drawn
from the same population. Think of all
the ‘isn’t it weird that Johnny and Sam-
my Smith are so different’ anecdotes–
many more, it seems to us, than ‘isn’t it
weird that Johnny and Sammy Smith are
so similar.’

If genes don’t ‘determine’ our behav-
ior, how can it be that obvious aspects
of our (or mice’s) environments don’t
entweder? We don’t know for sure, aber wir
can make some guesses. One is that re-
searchers have not yet identi½ed key en-
vironmental variables that are subtle to
them but central to a behaving organ-
ism–be it a mouse with a genome that
makes it love alcohol or Johnny trying
to get along with Sammy. Another is that
prenatal influences may put genetical-
ly similar (or identical) individuals on
quite different behavioral trajectories.
There is a tendency to think, ½rst there’s
fertilization, and then some nine months
later a baby pops out. Aber, Natürlich, ein
incredibly complex series of events takes
place during those nine months: cell-
cell, tissue-tissue, and organ-organ inter-
Aktionen; pulses of hormones; responses
to pleasant and unpleasant stimuli such
as voices heard through the uterine wall;
and in some cases interactions with
another fetus in the womb. Studies have
already shown what dramatic effects
prenatal environments can have. Für
Beispiel, young female fetuses whose
mothers had minimal diets during the
Dutch famine of World War II grew up
into women who were more obese than
those whose mothers were well fed; Sie
also had higher levels of ‘bad’ choles-
terol. As more is learned about environ-
mental influences in the womb it seems
likely that many of the differences be-
tween siblings could be discovered to
have prenatal origins.

Could there be another source of the
sometimes dramatic differences among

Dædalus Spring 2007

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