When Everyone Knows That Everyone Knows: Steven Pinker *****

In the past I've found Steven Pinker's books hypnotic - I drift along enjoying the words and feeling I'm getting something deep from it without anything much sinking in. This one was different - it was sometimes a little harder to get your head around the underlying concept, descending as it tends to into infinite regression, but overall I did feel I'd gained a lot. 

Pinker's focus here is 'common knowledge' which is when not only do we both/all know something, but we know that the other(s) know as well and the implications this has for human interactions.

One entertaining thing to emerge from this is the ability to get into infinite loops of the 'I know that you know that I know that...' variety. Although we aren't mentally equipped to deal with more than a handful of iterations, we are told that a simple but limited interaction like a text exchange strictly needs the full infinite set to be definitive. For example, if you and a friend are trying to choose between two possible locations to meet up, then you forever need a confirmatory text to make it clear that the other person received your text (and vice versa).

The topic also allows Pinker to bring in some entertaining if mind-stretching logic puzzles including the one that some consider the most challenging frequently presented. Because these interactions between people are often games in the game theory sense, there is also a considerable use of game theory in the book, which I also found entertaining.

One irritation from the game theory side is that the inevitable decision tables are really difficult to read in the way they are printed, because one player's values are in faint grey, hardly visible in my copy of the book. I also think that Pinker really should have given more introduction to what game theory is, where it came from and how these 'games' work - there's only a tiny amount of introduction and very little explanation. I do know game theory, but coming at it with no background, I would have found it unsatisfying.

I won't deny that there will still times that my brain went into 'not really taking this in' mode, and as almost always is the case with such psychology books focused on a particular point, it does occasionally feel like everything is being forced into a 'common knowledge' framework even if it doesn't fit awfully well. 

However, it's still a fascinating topic and I found the way that Pinker was able to take the often abstract-feeling game theory games into many real world situations thanks to their relationship with common knowledge human interactions that it was genuinely insightful. Interesting book indeed.

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Review by Brian Clegg - See all Brian's online articles or subscribe to a weekly email free here