The symbiont hypothesis of cooperation and Toxoplasmosis

I've previously promoted the symbiont hypothesis of cooperation and eusociality. As a brief summary, this proposes that sometimes agents cooperate because they are being manipulated into coming into contact by symbionts whose reproduction depends on interaction between their hosts. Here is my 2014 essay on the topic:

• Tim Tyler: Eusociality: the symbiont hypothesis

The theory has substantial significance for the evolution of eusociality in the organic realm. It also applies to the spread of cooperation in cultural evolution - based on the model that memes are the genes of cultural symbionts. Many memes promote cooperation partly because they need extended contact between humans in order for them to reproduce themselves.

One interesting illustration of the symbiont hypothesis involves Toxoplasmosis. Toxoplasmosis is caused by a microorganism that makes mice love cats - and is transmitted from mice to cats. It turns out that Toxoplasmosis also makes people love cats - and might be contributing to the plague of cat memes on the internet. Toxoplasmosis is an interesting example of a microorganism promoting love - partly because it is so well known. It makes a useful illustration of the symbiont hypothesis.

Update: for a sceptical take on the topic see here.