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PERHAPS because of its non-serious nature, play has been a neglected, even embarrassing subject for serious scientists. Little work has been done on what it’s for – that is, its evolutionary function. The Playful Brain has a stab at rectifying this, with some success.

Play is so complicated, say , that to understand it we need to build up a towering “layer cake” of information about neurology, physiology and juvenile development. Primates are difficult to experiment on – for this reason, and because of the authors’ area of expertise, we are plunged straight into a detailed analysis of play fighting in rats.

We have to wade through a lot of technical reading to get to the good stuff – and here I’m going to be shamelessly pro-ape – the parts about primates in general, and humans in particular. I enjoyed an anecdote about two juvenile gorillas play fighting, bashing into each other’s shoulders. One abruptly changes his attack, and grabs the other’s genitals, making him leap into the air in surprise.

So what is play actually for? Pellis and Pellis show that play improves social competence by honing the emotional and intellectual skills necessary to thrive in society. This explains why males play more than females: females are more socially competent to start with, while males by their nature are more aggressive and competitive.

The social competence theory prompts the authors to suggest that banning roughhouse play among children in many countries is not doing them any good. Preventing unstructured play is damaging – if you prevent an animal from playing when it is young, it will have serious social problems later. Yet extracting these nuggets was not the most fun I’ve ever had, and I’m left waiting for a really enlightening and engaging book on play.

Another attempt is . Like the Pellis’s book, there are some good anecdotes, but it seems to avoid the uncomfortable yet important area of childhood sexual play, barely mentions the evolutionary value of play, while seeming leaden and laboured.

In summary: all work and no play makes Jack a dull (and dysfunctional) boy.