Sex is Not a Disease

Traditional definition

To a geneticist, sex is normally defined as recombination with outcrossing [1].

Recombintaion means that different sets of genes are combined.

Outcrossing means that the different sets of genes can come from distantly-related individuals.


This definition has a problem - it fails to distinguish between sex and disease.

A DNA sequence that causes its bearer to grow a stalk that injects DNA into other organisms - in the hope of taking advantage of the genetic machinery in their cells - would normally be regarded as a virus.

However - by the traditional geneticists definition of sex - organisms so infected are sexual - since they wind up incorporating genes from distantly related organisms into their own genomes.

Sex and infectious disease are usually regarded as separate phenomena. They are associated with different adaptations, and one is considered highly undesirable - while the other is not.

Here I will argue that a definintion of sex that fails to distinguish it from infectious diseases is counter-intuitive, misleading and unfortunate.


How could sex and infectious disease be distinguished from one another?

Distinguishing sex from disease is not terribly easy. However, there are two main ways it could be done:

One way is to consider the frequency of the outcrossing. If all the members of the population do it call it sex. If only a few do it, call it an infectious disease.

The other approach looks at the effect on fitness. Sex is usually beneficial, whereas disease is typically harmful to all the genes - except the ones that cause the disease.

It is the second approach that I favour.


It is easy to see how the donor benefits from sex.

They get their genes copied without having to make any investment in constructing replication machinery or cell walls. This benefit is much the same whether the genes being passed on are useful to the recipient - or simply exploit the recipient's resources to further their own replication.

What is less obvious is how the recipient benefits from incorporating foreign genes into their genome.

I consider whether such a benefit exists to be the key feature which distinguishes sexual recombination from infectious diseases.

In the cases where there is no benefit - and the recipient's genes suffer - then the effect much like an infection - and it would be reasonable to expect adaptations to arise which favour disease resistance.

On the other hand, if the recipient's genes benefit from the encounter, then the effect is best classified as sexual in nature - and adaptations that assist the process are the likely result.


Some sexual acts may harm the recipient - but should still be considered to be sex - rather than disease.

Similarly, no doubt sometimes a disease will benefit an particular individual - perhaps via some resulting generous charitable donation which the sick individual would not have received if they had been healthy.

Rather than looking at each individual act - and classifying it according to whether it is beneficial, the usual - or expected case should be considered.

In other words, the key question is whether adaptations on the part of the recipient of the genes will favour pasting the genes in, or throwing them out.

The distinction being proposed here is similar to the one between consensual sex and rape.


A classification scheme which distinguishes between sex and disease seems useful.

One positive aspect about using a definition of sex that excludes disease is that the origin of sex is no longer the same as the origin of disease - and can be considered as a separate issue.


  1. Mitchod, Richard E. "Eros and Evolution"

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