Contraception, Abortion and Evolution: Population growth round two

Men who have lots of sex and indeed lots of different partners tend to have more offspring. For all of history and pre-history that has been the case. There have always been some second order effects such as, those with lots of different partners would have greater risk of contracting a venereal disease which can prevent further offspring being produced due to infertility or even death. On top of that, those who sleep around probably make more enemies. It is conceivable that they have a higher murder rate than their monogamous counterparts. Though I don't have anything other than anecdotal evidence to support this particular assertion.
Another issue is that the promiscuous males could loose their regular partners, who prefer to find a more faithful man.
However, in spite of all of that, more sex generally produces more offspring.

But in the last few decades the use of reliable contraception has become wide-spread, also in many countries abortion is readily available. In the past people generally had the choice to have sex or not. That led to pregnancies. Now there are two different choices, whether or not to have sex and it is a separate issue to choose to have kids. So the people who are having the most children are the ones who want to have them the most and not the ones who want sex the most. This is an enormous change as a driver of evolution. Also lesbians, or indeed any woman who can't find or doesn't want a male sex partner, can just go along the a sperm bank. So sex is not even a prerequisite to having kids.

New positive drivers that will lead to more children:
1: Following a religious organisation that discourages contraception and abortion. Perhaps the success of science in developing contraceptives will lead to a less scientific and more religious species.

2: Strong desire to have children. In the past, from an evolutionary point of view, it was very important to be able to attract a mate, have a desire to have sex and then the desire to look after the child when he arrived. Now there is a new issue, there needs to be a desire to have children, which clearly related to the desire to look after them once they have arrived, but they are distinct.

New negative drivers that will lead to fewer children:
1: Ambition in women to succeed in the world at the cost of having fewer children. A woman who doesn't want a career but just wants to start having lots of babies will clearly be more successful from an evolutionary point of view. In the past when there was more of a risk of starvation, the more worldly mothers could help provide better for her kids. Now with very high survival rates, having the kids in the first place is the key issue.

2: The alpha male types who never want to settle down, at least partially motivated by the fact that they have a constant supply of available females. In the past such men would have many offspring, even if they didn't stick round long enough to look after many of them. However, now with reliable contraception, the man who wants to settle down young have have lots of kids has an evolutionary advantage.

All of this is not just going to affect humanity in the future. To a certain extent, the effects have been seen already. For example in Northern Ireland, we saw a growth of the catholic population relative to the protestants at least in part due to the catholic prohibition on contraception.

Whether religiousness is down to nurture or nature, we can indeed witness that people who observe a rule that forbids the use of contraception have an evolutionary advantage.

We may well see population growth taking off once again as over the next few generations there will be a growth in the desire of people to have children because those who want to have children have a new evolutionary advantage.

On the other hand, the desire to look after children once they arrive is possibly now less important than it was previously, since in wealthy countries at least, social services often step in to look after children that have been neglected by their parents.

Cause of homosexuality

Evolution has to answer some tough questions, such as how could the human eye evolve (see Appendix 2 below) or why did peacocks evolve to have such a ridiculous (but beautiful) tail (see Appendix 3 below). In many respects evolution is a ruthless driver of efficiencies. But there are some anomalies. For example, why are there so many homosexuals? If there were a gene for gayness, then surely evolution would wipe it out. I'd like to proposes an explanation.


But first let me discuss tennis! ( It will be relevant, I promise)

When a tennis player is about to take a shot, naively we might think that the best strategy would be to hit the ball in such a way as to maximise the probability of it going in, that would be to hit it back gently into the middle of the court. However competitive players almost never do that.

If the player hits the ball harder (faster) then it will increase the probability of the ball going out, but if he were to gently hit the ball, then the opponent will almost certainly take advantage of the weak shot and hit a winner. So the optimal shot involves taking a calculated risk that the ball will go out. ( See Appendix 1 for a plot of some probabilities and expected score)

Now returning to evolutionary psychology: why are there so many gay people? Well, consider the following hypothesis: There are some genes which give people an advantage, such as say ability to recognise emotions in others. But rather than working just as a black and white (i.e. binary) scale, it is much more gray than that and that the genes can also lead to a small possibility (probability) of homosexuality.

Someone who carries these genes can still have an advantage over others who don't even though they expect some percentage of their offspring will be gay. Over all the effect of having the genes is positive.

Well, what could such a set of genes be? Or what quality do they give the people who have the genes. It could be a 'feminine-side'. A man who can express emotions and empathise with others who display their emotions can have an advantage over someone with very limited social skills. However having such genes could also lead to a probability of homosexuality. And so a genetic cause of homosexuality gets passed from one generation to the next in a bundle of genes with net beneficial effect.

The next question is how do we test this hypothesis? Well, in some families all the kids are tall. There is a high correlation in height of siblings. If the hypotheis is true, we would expect a lower correlation of homosexuality between siblings, since the genes are causing a probability of homosexuality and not actually homosexuality, unlike genes which cause height. But the correlation would still be positive.

Appendix 1
Let's look at the strategy of the tennis player in a mathematical way.

Let v be the speed of the ball that player A hits.
Ignoring drop shots for now, we'll consider speeds from the easy shot into the middle of the court up to the very high speeds which are very difficult to keep in.

Let p(v) be the probability that a shot hit with speed v by player A will go in.

Let q(v) be the probability that player B, on the other side of the net, will be able to return the ball again conditional on A's shot, which was hit with speed v, being in.

For both p(v) and q(v), low speeds will give high probabilities, high speeds will see the probabilities dropping towards 0.

For a given shot, we'll award A a score of

-1 when A's shot is out, that happens with probability (1-p(v))

0 when A's shot is in and B returns it successfully, that happens with prob: p(v) * q(v)

+1 when A shot is in and B is unsuccessful in returning it, that happens with prob p(v) * ( 1 - q(v))


We can now work out the expected score when A hits the ball with speed v:

E(S(v)) = -1 * (1 - p(v)) + 0 + 1 * p(v) * ( 1 - q(v))
E(S(v)) = 2 * p(v) - 1 - p(v) * q(v)


Below is a plot of what E(S(v)) might look like against v.



So we can see from the plot above, that to maximise his expected score, player A needs to hit the ball with a speed that will result in there being a reasonably high probability of the ball going out.

In exactly the same way, genes can be advantageous to the carrier, even if it means there is a significant probability of having homosexual offspring as long as they confer some other advantage.


Appendix 2
The evolution of the eye has been well explained. I won't go into the details here except to say that a useful introductory article can be found in wikipedia

Appendix 3
The best explanation I've heard for why peacock's beautiful tails have survived generations of ruthless evolution is that if a peahen were to have a preference for fit males with smaller tails, then her male offspring who have inherited the short tail from their father, would have trouble finding a mate. The cause of their problem would be that their mother had a opinion on which was the best mate that differed from the consensus. So, when choosing a mate, your children will thank you for going with the consensus.