Susan emphasizes that true behavioural imitation is hard - so hard that only a very smart primate mastered it. It requires the complex skill of putting yourself in someone else's shoes - in order to repeat their actions. Interestingly, this is almost the same ability that leads to empathy. For Susan, the difficulty of imitation is significant. SHe writes:
The first three processes alone will produce the selection pressures required to drive a runaway increase in brain size - if one further small assumption is made. That is that being good at imitation requires a big brain.By contrast, my idea is more that ancestral human crainia were completely filled with memes. Memes were on average beneficial - and the more of them you had space for the fitter you were.
In my book, I compare the human skull to ant domatia, figs and Alder tree root nodules - all structures produced by plants to house symbionts. The human skull houses cultural symbionts - and that is the main reason why it grew so dramatically over the last three million years.
For a while I didn't realise that I had mutated her idea. I thought I had got my idea directly from her. Later, when I reread The Meme Machine I noticed that there were some differences between her idea and mine. Some mutations had taken place in my brain.
I'm not saying that Susan's idea is wrong - but I still like my take on the idea more. One of the virtues of my idea is that it is pretty specific - which should help with testing it. The idea of the big brain as a meme nest does not require Susan's additional "small assumption" that "being good at imitation requires a big brain".
Since practically every difference between us and our nearest primate relatives is down to our cultural symbionts, it is no big surprise that our big brains are too. However - as far as I know, Susan Blackmore was the first person who really took this idea seriously and promoted it.
I find it distressing to see the extent to which her contribution has been written out of scientific history. We have many modern papers on the idea that the enlargement of the human brain was caused by cultural transmission [see references here]. Few or none even mention Susan Blackmore's contribution. It seems like a revisionist history.