Twins - Lawrence Wright

Kinds of Minds - Daniel C Dennett

Living With Our Genes - Dean Hamer

The Sickening Mind - Paul Martin


Twins pose questions we might not think to ask if we lived in a world without them. They are both an unsettling presence, because they undermine our sense of indivudual uniqueness, and a score-settling presence, because their mere existence allows us to test certain ideas about how we are the way we are. Every culture has had to confront the twin phenomenon and come to its own response.

Using twins, and data dervied from adoption studies, scientists can now estimate what proportion of the variation in our intelligence, our personality, our behaviour, and even seemingly random life events such as bankruptcy or the death of a spouse, might be caused by inherited tendencies.

The prevailing view of human nature at the end of the century resembles in many ways the view we had at the beginning. That is that people are largely responsible for their station in life, and that circumstances do not so much dictate the outcome of a person's life as they reflect the inner nature of the person living it. Twins have been used to prove a point, and the point is that we don't become. We are.

The heritability for height among white European and North American populations is about 90%.

When the University of Minnesota team studied religious interests, attitudes, and behaviours of twins reared apart, as well as twins reared together, the found that genetic factors accounted for about 50%. Religious affiliation on the other hand - one's denomination or belief - was laregly environmental. As the team extended their studies of personality, a pattern emerged : characteristically, about half the variance in most measurable personality traits turned out to be genetic.

The role of heredity in determining intellignce only seems to grow with age.

With the increasing use of ultrasound to detect early pregnancies, we now know that twinning is a far more common occurence than anyone had previously imagined. Although only one out of eighty or ninety live births produces twins, at least one-eight of all natural pregnancies begin as twins. Most of us singletons, in other words, began life as something more - as part of a pair.

In every twin pair, one is usually considered more attractive than the other. By having students pick the more attractive twin from various pairs of photos, David Teplica, hopes to measure the anatomic variances that make up the standards of beauty.

Charles Boklage believes that most malformed children who were born as singletons actually may be the products of twin pregnancies. "It is possible that I am twins. By that I mean that two different embryos went together to make one body. I can tell you with complete certainty that some of us are twins who are walking around in a single body."

In Sweden, where all adoptions are recorded for national statictics, a 1982 study found that the rate of criminality in adopted children was 2.9% when neither biological nor adoptive parents had committed a crime; the figure rose to 6.7% if their adoptive parents were criminal and 12.1% if their biological parents were criminals. If both sets of parents were criminal, however, the chances of the child being criminal as well were 40%. Together, genes and environment appear to be several times more compelling than either force acting alone.

Sandra Scarr found that (adopted) children in the same family who were genetically unrelated were alike in early years but grew to be different over time. They became more like their biological parents, whom they didn't know, than like the adoptive parents who raised them. As long as children in a population are reasonably nurtered, Scarr observed, the individual differences between them must be genetic.

As children mature, they gain more and more control over their environment, and actively select from the super-abundance of opportunities those which conform to their genetic disposition. Scarr wrote : "We propose that development is indeed the result of nature and nurture, but that genes drive experience. Genes are components in a system that organises the organism to experience the world."

If genes account for half the development of the personality, then environment must account for the remainder. What, exactly, in the environment shapes personality? The answer is that the common shared environment - the family, the neighbourhood, the parents' income and level of education, their way of raising children - has essentially no effect on the development of personality.
Identical twins who have been reared apart are not much different in various personality measurements than twins rearer together. It is the individual experiences each person has, such as the education he receives, the friends he makes, birth order, accidents, - the unshared environment - that accounts for nearly all the personality difference than can be ascribed to non-genetic factors.

Divorce also looks like a genetic disease. If an identical twin gets divorced, the chances that his co-twin will become divorced is about 45%, for fraternal twins, the chances drop to 30%. That difference strongly suggests a genetic component to the risk of divorce.
The genetic risk of divorce is a feature of many inherited personality traits, such as impulsiveness and neurotic behaviour. Genes that create havoc in the parents' lives are likely to do the same in the lives of their children.

The entire process of evolution is one of genes adapting to the environment. In the larger sense, we do not make the environment, it makes us.


Their (twins) history affords a means of distinguishing between the effects of tendencies received at birth, or those that were imposed by the circumstances of their after lives; in other words, between the effects of nature and nurture.

- Francis Galton (1869) Heritability is the fraction of the observed variation in a population that is caused by differences in heredity. - JL Lush (1940) It's now clear to me that almost all secondary skin characteristics on the head and face, and probably elsewhere, are genetic predetermined. For example, freckle patterns, hair whorls, the first grey hairs, the first wrinkles on the human face, even the development of acne on the same location on the nose at exactly the same time - all these things seem to be in some way genetically predetermined.
How can it be that two cell clusters that were separated fifty years ago have enough information to determine where your blackheads would develop when you are fifty or sixty years old? - David Teplica Nature is often hidden, sometimes overcome, seldom extinguished. - Francis Bacon Quite clearly its crass to equate genetics with determinism and environmentalism with freedom. I think human freedom means something about the capacity of the human organism not to be pushed around or dominated by external circumstances. I would argue that evolution has given us our freedom. Freedom is the ability to stand up and transcend the limitations of the environment. - Lyndon Eaves #


I am a philosopher, not a scientist, and we philosophers are better at questions than answers. I haven't begun by insulting myself and my discipline, in spite of first appearances. Finding better questions to ask, and breaking old habits and traditions of asking, is a very difficult part of the grand human project of understanding ourselves and our world.

Can we ever really know what is going on in someone else's mind? Can a woman know what it is like to be a man? What experiences does a baby have during cbildbirth? What experiences, if any, does a fetus have in its mother's womb? And what of nonhuman minds? What do horses think about? Why aren't vultures nauseated by the rotting carcasses they eat? When a fish has a hook sticking through its lip, does it hurt the fish as much as it would hurt you, if you had a hook sticking through your lip? Can spiders think, or are they just tiny robots, midnlessly making their elegant webs? For that matter, why couldn't a robot - if it was fancy enough - be conscious?

Are we so sure that all human beings have minds? Maybe you're the only mind in the universe; maybe everything else, including the apparent author of this book, is a mere mindless machine. This strange idea first occured to me as a child, and perhaps it did to you as well. Its such a common philosophical hypothesis that it has a name - solipsism ( from Latin for myself alone).

Membership in the class of things that have minds provides an all-important guarantee: the guarantee of a certain sort of moral standing. Only mind-havers can care; only mind-havers can mind what happens. If I do something to you that you don't want me to, this has moral significance. It matters, because it matters to you. It may not matter much, or your interests may be overridden for all sorts of reasons, or if I'm punishing you justly for a misdeed of yours, the fact that you care may actually count in favour of my deed.
In any event, your caring automatically counts for something in the moral equation. If flowers have minds, then what we do to flowers can matter to them, and not just to those who care about what happens to flowers. If nobody cares, then it doesn't matter what happens to flowers.

To underattribute minds - to disregard or discount or deny the experience, the suffering and joy, the thwarted ambitions and frustrated desires of a mind-having person or animal - would be a terrible sin. If we overattributed minds (if, for instance, we got it into our heads that since bacteria had minds, we couldn't justify killing them), this might lead us to sacrifice the interests of many legitimate interest-holders - our friends, our pets, ourselves - for nothing of genuine moral importance.

Before our ancestors got minds, they got bodies. First, they became simple cells, or prokaryotes, and eventually the prokaryotes took in some invaders, or boarders, and thereby became complex cells - the eukaryotes. By this time, roughly a billion years after the appearance of simple cells, our ancestors were already extraordinarily complex machines (made of machines made of machines), but they still didn't have minds.

One of the fundamental assumptions shared by many modern theories of mind is known as functionalism. What makes something a mind is not what it is made of, but it can do. What makes something a spark plug is that it can be plugged into a particular situation and deliver a spark when called upon. A heart is something for pumping blood, and an artificial heart or a pig's heart may do just about as well.
Why couldn't artificial minds, like artificial hearts, be made real - realized - out of almost anything? Once we figure out what minds do (what pains do, what beliefs do, and so on) we ought to be able to make minds out of alternative materials that have those competencies. And it has seemed obvious to many theorists - myself included - that what minds do is process information.

Can a dog figure out how to unwind his leash when he runs around a tree or lamppost? But few if any dogs can. And dolphins, for all their intelligence, are strangely unable to figure out that they could easily leap over the surrounding tuna net to safety. As researchers regularly discover, the more ingeniously you investigate the competence of nonhuman animals, the more likely you are to discover abrupt gaps in competence. The ability of animals to generalize from their particular exploitations of wisdom is severely limited.

The psychologist Nicholas Humphrey argues that the development of self-consciousness was a stratagem for developing and testing hypotheses about what was going through the minds of others. One uses one's self-consciousness as a source of hypotheses about other-consciousness.

A first order intentional system has beliefs and desires about many things, but not about beliefs and desires. A second order intentional system has beliefs and desires about beliefs and desires, its own or those of others. A third order intentional system would be capable of such feats as wanting you to believe that it wanted something, while a fourth order intentional system might believe you wanted it to believe that you believed something and so forth. The big step is the step from first to second order.

Consider, for instance, the many different methods we have developed for thinking about time by actually thinking about space. We have all sorts of conventional ways of mapping past, present and future, before and after, sooner and later - differences that are virtually invisible in unrefined nature - onto left and right, up and down, clockwise and counter-clockwise.
Monday is to the left of Tuesday, our spatialization of space extends to graphs - think of the profits, or the temperature rising up up up from left to right with the passage of time. We use our sense of space to see the passage of time.

Does a dog have a concept of cat? Yes and no. You and the dog discriminate the same sets of entities as cats and noncats, but the dog cannot consider the concept. It cannot ask itself whether it knows what cats are. Concepts are not things in the dog's world in the way that cats are. Concepts are things in our world, because we have language. No languageless mammal can consider snow 'in general' or 'in itself'.

A dissociated child does not suffer as much as a nondissociated child. But now what should we say about creatures that are naturall dissociated - that never achieve, or even attempt to achieve, the sort of complex internal organization that is standard in a normal child and disrupted in a dissociated child? An invited conclusion would be: such a creature is constitutionally incapable of undergoing the sort or amount of suffering that a normal human can undergo. But if all nonhuman species are in such a relatively disorganized state, we have grounds for the hypothesis that nonhuman animals may indeed feel pain but cannot suffer the way we can.

This book began with a host of questions, and since this is a book by a philosopher it ends not with the answers, but, I hope, with better versions of the questions themselves.



Personality is so complex that even though millions of people have walked the Earth, no two have ever been the same.

You look like the people in your family - and in some respects you feel and act like them too. You have about as much choice in some aspects of your personality as you do in the shape of your nose or the size of your feet.

You cannot be anything you want to be, but it is possible to be all you can be.

Every fetus is created without a sex until a single gene switches on and begins a cascade of chemical reactions that turns half of us into males and half into females.

Numerous studies show that altering the levels of a single brain chemical can completely change an animal's level of aggression. A simple manipulation of a single gene can turn a meek and mild rat into a crazed killer. The same brain chemicals are found in humans.

We accept that we look like our parents and other blood relatives; we have a harder time with the idea that we also act like them. In other species, we value and encourage genetic differences in "personality". Consider the difference between a Wisconsin dairy cow and a bull from Pamplona, or a golden retriever and a pit bull. Human breeding is less orderly, but children do share personality traits with their parents. This is not bad; it's beautiful. This does not mean we are doomed to become our parents; it means we begin our journeys where our parents left off.

Those who accomplish the most - measured in money, intelligence, skill, happiness, or love - are the ones who make the most of their genetic inheritance. Genes determine the range of what is possible.

The brain and body are built by DNA, and everyone's DNA is pretty much the same. We all have 99.9 percent the same DNA as Michael Jordon, Albert Einstein, Elizabeth Taylor, Charles Manson, Julius Caesar, Julia Child, and Jules Verne.

Scientists looked at alcoholic fathers who put their sons up for adoption. Both in the United States and Sweden, the biological children of alcoholics had a four times higher risk of becoming alcoholics themselves than the children of nonalcoholics., even though they'd been separated from their alcoholic parents after the first few weeks of life. When the experiment was reversed - when the children of nonalcoholics were adopted by alcoholics - the children had no increased risk for alcoholism, even though they were exposed to it in the home.

If two boys are raised in the same home and one turns out gay and the other straight, the difference was not caused by the general parenting style.

No matter in what country, in which historical time period, in what age group, or with what type of test, the result is always the same: no other single factor is more important than genes in determining cognitive ability.

1 out of every 100 Americans can't lead an independent existence because a genetic defect has stunted their mental ability.

The most recent experiments show that genes are the single most important contributor to body weight, more than any other factor or combination of things. The best estimate is that body weight is about 70% inherited.

Our obsession with being thin actually contributes to making some people fatter. The reason is that a drastic or crash diet causes the body to fear starvation. The natural, physiological response to a sudden shortage of food is for the fat cells to decrease the production of leptin. This sends a powerful bodily signal to eat more and to conserve calories, which is why yo-yo dieting not only fails too keep weight off, it can result in weight gain.

Genetic manipulation of behaviour is not new. Humans have been selectively breeding behaviour in animals, such as dogs and farm animals, since before history. And whether we recognise it or not, we already are products of genetic engineering. As far back as we can see, humans have always been selective about their mates. Beauty, power, and prestige have always been desirable.



During the Gulf War in 1991 Iraq launched a series of missile attacks against Israel. Many Israeli civilians died as a result of these attacks. But the vast majority of them did not die from any direct physical effects of the missiles. They died from heart failure brought on by the fear, anxiety and stress associated with the bombardment.

There is less likelihood of a person dying on the eve of an occasion that has symbolic significance for them, such as an important religious festival or birthday. There is compelling evidence that individuals on the verge of death can postpone their death for a few days until the special occasion has passed.

Diseases of the heart and circulatory system, cancer and accidental injuries now account for more than three-quarters of all deaths. In contrast, infectious and parasitic diseases account for less than 0.5 percent of all deaths.

Not everyone who is exposed to disease-causing bacteria or viruses becomes infected. Not everyone who gets infected with disease-causing viruses or bacteria develops a clinical disease.

All the leading causes of death in industrialized nations - including heart disease, cancer, accidental injury and AIDS - depend to some extent on how we behave.

Smoking is the riskiest thing that most people will ever do in their lives. Take a random sample of a thousand young men who smoke; on the basis of actuarial data it can be confidently predicted that one of these young men will eventually be murdered, six will be killed on the roads and two hundred and fifty will die prematurely from the effects of smoking.

Merely thinking about something unpleasant can itself be stressful.

Individuals differ considerably in their response to identical stressors, depending on factors that are unique to them. A situation that is highly stressful for one person may be of little or no consequence - or even be positively invigorating - for someone else. Stress, like beauty, is partly in the mind of the beholder.

Research on American soldiers during the Vietnam War found that the level of stress hormones in certain servicemen was lower during active combat than when the men were off duty. This was especially true among highly trained, elite units with a strong sense of group cohesion. The harassed accounts clerk really can have a worse time of it than the marine commando.

The fundamental purpose of the stress response is to enable organisms, including you and me, to cope swiftly and effectively with life-threatening challenges... it involves a rapid switch of priorities from long-term to short-term survival.

If things get really terrifying the parasympathetic nervous system can trigger involuntary urination and defecation. Besmirching yourself in this way might be messy, but having an empty bladder and bowel could be helpful if things get desperate. It makes you lighter and, perhaps, somewhat less appetizing to a potential predator.

When a stressor occurs just before exposure to pathogens it can enhance immune function and increase the organism's resistance to disease. If, however, the stressor occurs during or soon after exposure to pathogens it can impair immune function and make the organism less resistant to disease.

The predictability of a stressor can also have a bearing on its biological impact. By and large, being able to anticipate a stressor shortly before it arrives is a Good Thing. Predictability also lessens the impact of stressors on immune function.

The impact of any particular stressor depends to a considerable extent upon the degree to which it can be controlled - that is, the extent to which the person on the receiving end has the power to alter, eliminate or escape from the stressor. We exert control over stressors by performing any behavioural response which enables us to escape from the stressor, terminate it, or recude its severity.

Within limits, we perform better when we are mildly stressed. Among students taking an academic exam, those who exhibit the largest increases in adrenaline tend to get the best exam results.

Performance and stress typically have an inverse U-shaped relationship: extremely high or low levels of stress have a distinctly detrimental effect, but intermediate levels go hand in hand with optimal performance.

Mild, short-term stress can enhance rather than suppress the immune system.

In the 1960s scientists found that when baby rats or mice are repeatedly exposed to a mildly stressful experience, such as being handled by a human, they grow up to be more stress-resistant as adults. All in all, mild stress in early life seems to make them better able to cope with stress later in life.

Individuals who reach the senior ranks of their profession are (in theory, if not always in practice) more capable of dealing with larger demands, by dint of experience if not natural ability. They have already been selected, in part, for their ability to cope with the difficulties, demands and stressors, of their work. You are unlikely to make it to the upper echelons if you crumple under the normal pressures of work.

If you think having to slave away in a crummy job is bad for your health, rest assured that being unemployed is unhealthier yet. The very threat of unemployment can be enough to harm you.

The underlying incidence of most cancers has not changed hugely in recent decades, although the absolute number of deaths from cancer has shown a marked increase, primarily because the elderly now live longer and account for a greater proportion of the population. We all have to die of something.

Many relaxation techniques appear to exploit the phenomenon of conditioning (associative learning). During the training period the individual unconsciously learns to associate a previously neutral stimulus, such as a particular sound or thought or smell, with the sensation of mental and physical relaxation. By repeatedly pairing the neutral stimulus with the relaxation, the stimulus is eventually capable of eliciting the relaxed state by itself. All the individual need do is croon the mantra, imagine the scene, smell the aroma, take the deep breath or hear the piece of music and the conditioned response of mental and muscular relaxation automatically follows.

Development is a truly extraordinary process. Each of us starts life as a single fertilized cell containing 46 double strands of DNA, along which are arranged approximately 100,000 genes. Years later, by means that are as yet only dimly understood, we each end up as a distinctive individual with a body consisting of over ten million million specialized cells. Each of those ten million million cells is in the right place and doing the right things, and as a result each one of us is unique.

The current consensus is that natural selection has favoured the evolution of fever because it helps those who are sick to combat infection. Fever is designed to make life harder for the offending bacteria or viruses. Like all organisms, bacteria and viruses work best at a particular temperature. Alter that temperature and they cease to thrive. A corollary of this evolutionary explanation is that taking drugs to 'cure' fever might do more harm than good.

According to biologist Margie Profet's theory, morning sickness is not merely an annoying by-product of pregnancy; rather it evolved to deter pregnant mothers from eating or drinking toxic substances that might harm the foetus.

What all these examples - coughs, fever and morning sickness - illustrate is that we dismiss medical phenomena as mere symptons at our peril. They may be manisfestations of biological defence mechanisms which have evolved for a reason and which have benefits as well as costs. We would do well to understand the nature of these benefits before we tak drugs to suppress them.

As far as natural selection is concerned, what happens to your health after you are too old to reproduce or care for your offspring is laregly irrelevent.

Genetically, you and I are almost identical to the humans who were around at the dawn on civilization. As George Williams and Randolph Nesse have pointed out, a hunter-gatherer baby born thousands of years before they wheel was invented would be biologically capable of growing up to become a lawyer or an accountant.

We must expect to be on the losing side in the evolutionary arms race at least some of the time. Bacteria and viruses have a huge advantage over humans because they are minute, multitudinous and prolific reproducers. No matter how fastidious you are about your personal hygiene, there are more bacteria inhabiting your body than there are human beings on the planet. And those bacteria are reproducing several hundred thousands times faster than you could manage. Humans take twenty or thirty years to produce a new generation. Bacteria can do it in thirty minutes. Under the right conditions, they can whizz through tens of generations in the course of one day, reaping evolutionary advantages that would take us hundreds of years to achieve. Evolution by natural selection is going on inside your body right now.

Fifty years ago there were no penicillen-resistant strains of staphylococci (the bacteria responsible for many post-operation infections). Now, nearly all staphylococci are resistent to penicillen and a range of other antibiotics besides. Much the same has happened with the drugs used to combat malaria. Natural selection has produced resistant strains of the protozoan parasites which cause malaria, rendering one after another of the standard anti-malarial drugs ineffective.

Evolution by natural selectionis, to repeat the familiar refrain, all about reproductive success and the differential survival of genes.

The human organism is the most complex and extra-ordinary entity in the known universe.


Return to Quotes index, or Site homepage.