by Rik Smits
Like hand preference, sexual preference can be divided, roughly speaking, into two categories, in the same ratio of one to nine. The majority of us are attracted to the opposite sex, while 10 per cent of us prefer people of the same sex. As with hand preference, the division is not razor-sharp and absolute, but most people are preponderantly one way or the other. It seems as if in sexual orientation there are more cases of explicitly mixed preference, but appearances may be deceptive. There are for example people who label themselves bisexual for ideological reasons. As with the number of left-handers, the percentage of homosexuals, male and female, seems to have been more or less constant for many centuries, all over the world. There is one clear difference, however. While it remains an open question whether hand preference is linked to any evolutionary advantage or disadvantage, there can be no doubt that homosexuality is a textbook case of a trait that makes people less likely to reproduce.
A confirmed homosexual has no interest in intercourse with the opposite sex and therefore in Darwinian terms he or she fails to compete. In fact from the point of view of both natural selection and sexual selection, a homosexual lifestyle is so disastrous that it’s mystifying how this trait ever became established and why the numbers have remained constant even in the face of outright persecution.
We need to note at this point that the repression of homosexuality may actually have contributed to its survival. Throughout history homo-sexuality has been viewed with almost universal contempt, or indeed simply denied and oppressed. It was at best something engaged in by men at the margins of society. We need only look at the way people in strictly sexually segregated societies today turn a blind eye to homosexual contact as a safety valve for frustrations among young men. All men, homosexual or not, were expected to start a family and care for their children. Women were given no choice; they were simply presented with some equivalent of the Victorian adage ‘close your eyes and think of England’. In many societies, sadly, they still are.
So for millennia most homosexual men and women had little chance of finding fulfilment, if we assume this would have involved sticking to relationships that suited their sexual orientation. They were forced to engage in family formation and reproduction to a more or less normal degree. The situation is probably little different today, not only in countries where homosexuality officially does not exist but even in Western Europe and America. So the negative effect on reproductive success has probably been considerably reduced by social pressures. It’s far from negligible all the same. Which of us doesn’t know at least one confirmed bachelor, not to mention a middle-aged man who has never flown the parental coop, house-sharing sisters, elderly spinsters or even a nun or a hermit? These are all lifestyles that have traditionally provided unsuspected, if perhaps unfulfilled homosexuals with a way to live.
As a self-stabilizing trait like left-handedness, homosexuality could easily acquire a place for itself within the human population. In fact as soon as it existed in a few individuals, for whatever reason, it would inevitably acquire such a place. The negative effect on reproductive success does not alter this, it only means that a slightly higher average number of children need to be born, across the board, to maintain the population level.
Take a concrete example. We begin with the same values as those for left-handedness, in other words a basic probability of homosexuality in any given child of roughly one in ten, double that if one of the parents is homosexual. We then assume that all pairs made up of two homosexuals fail to reproduce at all and that half of all couples of which one partner is homosexual are also childless. These are strict conditions, and they reduce reproductive chances considerably, but even then the average number of children per couple needed to maintain the population at its existing level only rises from two to 2.25. The number of homosexuals in the population will remain absolutely steady at a little over 11 per cent.
How homosexuality stabilizes in an initial population of 10,000 people, given that the basic likelihood of homosexuality is 10%, each homosexual parent doubles that likelihood, homosexual couples do not have children and only 50% of couples of which one partner is homosexual reproduce. On average each couple has 2.25 children. Whether the percentage of the population that is homosexual is 98 or only 2 at the start, within about five generations the percentage reaches 11. Room for variation lies only in the size of the resulting population.
It’s more than possible that there are other traits of this kind, occurring at some constant frequency through all times and all circumstances, immune to selective pressures. We don’t know of any, but that could be because they are uninteresting or inconspicuous, or occur invisibly, deep inside our bodies. They need not have anything in common, just as there is no connection between sexual orientation and hand preference, so they may have quite different causes and even rest on quite different principles. What those principles are is of course a mystery, but in the case of left-handedness we can have a go at identifying the driving force that lies behind the phenomenon, based on the remarkable patterns found in twins.
32
Left-handers as Undercover Twins
In 1967 the psychologist A. Subirana sighed that it almost seemed as if left-handed people were created purely to sour the lives of neurologists, but we might be equally justified in saying that twins were invented to sabotage explanations of left-handedness. Whatever approach we take, there they are, blocking the path.
Still, maybe there’s a lesson here, an indication that we’ve always looked at twins the wrong way. Twins are a special group, so special that we’re used to seeing them as the exception to the rule. What if that’s not in fact true when it comes to left-handedness? What if twins, however rare, are actually the norm and there’s something special about the left-handed product of a single birth? This feels like skating on thin ice, but it’s an alluring idea nonetheless. Especially if for the time being we limit ourselves to looking at monozygotic twins.
There are at least three facts we can take as a starting point. The first is that left-handedness is almost twice as common among twins. This in itself is old news, but it becomes interesting when we combine it with a second fact: left-handed people seem to be roughly twice as likely to produce twin offspring as their right-handed brothers and sisters. This means there’s a link both between twins and left-handedness and between being the parent of twins and left-handedness.
The third fact is that a left-handed person is roughly twice as likely to have a left-handed child as a right-handed person. If we take the liberty of combining this with the second fact, then we see that the chances of left-handers having twins and of having left-handed children are greater than normal to roughly the same degree. This looks promising, especially if we take into account that the incidence of monozygotic twins as a proportion of births varies hardly at all from one community to another, just like the proportion of left-handers.
Now we have to go one step further and make a bold assumption. Suppose not just that there’s a connection between twins and left-handedness but that being a monozygotic twin is a precondition of being left-handed, such that only embryos that divide at an early stage can develop into either one or two left-handed foetuses.
The appeal of this idea lies in the fact that we then need only one characteristic to be passed down in some way from generation to generation in order to explain both non-pathological left-handedness and the existence of twins. It’s clear what such a trait would have to involve: a stronger or weaker tendency for the early embryo to split. If a mother’s body naturally exerts a constant pressure on that initial clump of embryonic cells to divide, and if that pressure is greater in the case of left-handed parents, then we come satisfyingly close to what our experience with twins and left-handed people already teaches us. We’ve found ourselves a theoretical trait that in every relevant sense causes hand preference and has some features of an inherited characteristic without being directly dependent on the presence of specific gene variants, which would have to occur in
a precise, stable proportion of the population. But first we need to find our way around a couple of obstacles.
Even before we’re properly out of the starting blocks with this new theory we come up against the incontrovertible fact that twins are far less common than left-handers. One in ten births produces a left-handed child, but monozygotic twins arise in only three or four cases in every thousand. To see how we can reconcile these figures, we first have to look more closely at what the creation of twins actually involves.
Monozygotic twins start out as such in the first eight or nine days after conception. It’s then that a little bundle of cells, just starting to develop into an embryo, can sometimes split into two roughly equal parts, each of which is capable of growing into a complete and independent human being. That early embryo, although minuscule, is already fairly complex. The popular image of the fertilized ovum that immediately splits into two parts, roughly in the way an ordinary cell divides, is incorrect. That image arose out of famous experiments with salamander eggs, which when cut with a hair can develop into two new eggs, producing two viable individuals. Human twins generally arise at a much later stage.
On the fourth day of pregnancy a membrane known as the chorion grows around the clump of cells. In two out of three cases of monozygotic twins it surrounds both embryos, so clearly some two thirds of twins develop after this stage. Twins that have already split by this point each develop their own chorion. Around the seventh day another membrane develops inside the chorion, known as the amnion. This is hardly ever shared by twins, so by that time the split must have taken place. The majority of twins, therefore, must develop between the fourth and the seventh day after conception. The rare cases in which twins separate so late that they share the amnion as well as the chorion often involve unpleasant outcomes. They include all conjoined twins.
This relatively late splitting of cells means that the formation of twins is a fairly complex and risky process. By comparison, when a single cell splits little can go wrong. Division is something that comes naturally to most cells, but a multi-cellular structure is not easy to split in such a way that both parts fulfil all the requirements for further development. It is therefore an open question how Mother Nature gets away with it now and again.
Let’s answer that question in the simplest way possible, by assuming that no special procedure is involved: if an embryo splits, it does so in a random manner. In the vast majority of cases one part will have insufficient material to develop further. It’s a formless clump of a few dozen cells at most, so it can easily disappear without trace. This may seem inefficient, but it has the beauty of simplicity and it takes us a long way in the right direction, since it means that most children who at the beginning of pregnancy are one half of a pair of twins come into the world as single births.
You might think it ought to be easy to find out whether there’s an element of truth in this idea, but that is not so. We’re talking about an event in the first few days of a normal pregnancy. Future parents usually have no idea at this stage that they have conceived a child (leaving aside the various artificial ways of creating a pregnancy, which aren’t of any help to us here). Moreover, even if we could get there in time to observe the course of events, what mother-to-be would be willing to make herself and her tiny embryo available for research from which neither she nor her child have anything to gain? It’s no wonder that despite all the modern technology at our disposal, the development of monozygotic twins is still shrouded in mystery.
Time to go back to our starting point: left-handedness arises through the splitting of an embryo. A further indication that we are on the right track is that this splitting occurs precisely at the stage when the foundations are laid for the symmetries and asymmetries of the developing baby. An early embryo may be no more than a slimy bundle of cells, but it is rapidly developing the fundamental distinctions between back and belly, bottom and head. It could hardly be otherwise, since within about three weeks of conception the basic structure of a little human being is in place, with a tiny spinal cord, the very beginnings of different kinds of organs and muscles, a primitive system of blood circulation and even a heart. This is the tadpole stage in which the embryo is about one centimetre long.
Occasionally an inconsequential asymmetry in one of a pair of monozygotic twins will occur in the other as well, but in reverse. A well-known example is the shape of the ear, which on one side of a twin sometimes has precisely the same contours as the ear on the other side of the other twin. Further examples of mirror-imaging of this kind are eyelid shape and scalp hair-whorl direction. This could be a result of disruption to the calm, normal course of events, caused by the splitting of an embryo just when the first structural rudiments are being laid down. Thinking along the same lines, we might hypothesize that we are all theoretically destined to be right-handed but there’s a chance that splitting will disturb an initial impulse towards the formation of asymmetrical features in the brain, so that one of the two embryos becomes left-handed, or even both.
A model of a human embryo at roughly three weeks old, made by Colin Quilter of the University of Auckland in New Zealand. On the left the model shows only the neural tube that will form the spinal column, with the nodule at one end that will develop into the brain. On the inner side of the tube is the notochord, a primitive spinal column. On the right is the complete model, with the blood circulation and a tiny heart (the balloon-like structures at the centre). Across the back are the somites, segments of tissue from which the skin, vertebrae and muscles will develop. This stage is reached within fourteen days of the last possible moment when viable twins can be produced.
It’s important to remember that this can be no more than a slight disruption to the normal development of asymmetries if splitting is to have a happy outcome at all. We’re dealing with a minuscule, extremely fragile scrap of tissue. Fortunately, if the process of division proves fatal, most would-be parents will never know the pregnancy existed.
We should not be surprised that significant reversals are rare, nor that where they do occur, as in the case of partial or complete situs inversus, they can cause serious problems. In the brain, which at the time of splitting has barely begun to develop, nothing too strange can be allowed to happen if the embryo is to survive. So the brains of left-handers usually look barely any different from those of right-handers and the most important functions remain in their usual places. In a minority some degree of reversal takes place, but as far as we can tell by the means now available to us, fMRI in particular, the most common effect seems to be that the brains of left-handed people are slightly less lateralized. Intuitively at least, this fits rather well with the idea that a minor ripple in early development is responsible, caused by the splitting of the embryo.
Based on what little we know with reasonable certainty, we can have a fair crack at calculating how great a tendency embryos have to split. The chance of left-handedness in any given individual is around 10 per cent. With twins the likelihood that both will be right-handed is about 64 per cent, while the likelihood that one will be right-handed and one left-handed is roughly 32 per cent. Anyone who’s good at arithmetic can easily check all the sums, but for now we’ll put the precise percentages to one side to avoid being distracted from the overall picture. Suffice it to say that they coincide neatly with the result that pops up again and again in all manner of research: left-handedness occurs in roughly one third of pairs of twins. The likelihood that at least one twin will be left-handed is slightly higher, at 36 per cent, since that figure includes the rare cases where both twins are left-handed.
It has also been shown that there’s a chance of around three or four in a thousand that conception will result in a viable pair of monozygotic twins. Let’s call it four. If 100,000 conceptions take place, they will produce 100,400 individuals, since 400 embryos will have successfully doubled to become twins. One in ten individuals are left-handed, so these 100,000 conceptions will produce 10,040 left-handers. Assuming they were all
the result of the splitting of an embryo, we can calculate how many such splitting events must have occurred as a minimum. That number is equal to the number of left-handers divided by the likelihood of a pair of twins being produced with at least one left-handed individual. Dividing that result again by the total number of 100,000 conceptions gives us the likelihood that an embryo will split. It comes to a little under 28 per cent.
Since we know that out of every thousand conceptions only four actually produce twins, we now also know the likelihood that the splitting of an embryo will produce two viable individuals: a little less than 1.5 per cent.
Now we can work out how parental characteristics influence the tendency of their embryos to split. Twins are so rare that we can afford to ignore the influence of twin parents, but not of parental hand preference. Any embryo taken at random has an 81 per cent likelihood of having been conceived by two right-handers. Eighteen per cent have parents with different hand preferences, and 1 per cent are conceived by two left-handers. If we combine this data with the fact that each left-handed parent doubles the likelihood of splitting and twin formation, then we can conclude that with a right-handed couple, 23 per cent of embryos split, with mixed couples 46 per cent and with couples composed of two left-handers no fewer than 92 per cent.
Taking into account that the initial data are only approximate – the percentage of left-handers, for example, is not precisely ten; the chances of a left-handed child being born to a left-handed parent are not precisely double the norm – we can say that with right-handers a quarter of embryos split and with mixed pairs half, while the embryos of completely left-handed couples almost all split. This pattern must be the same when it comes to the chances of twins being born to parents one or both of whom is a twin.