So the rock-hard-evidence-based message of Bowling Alone is that social involvement has been declining drastically in recent generations. Or rather, social capital has been declining. One line definition: Capital means stuff of value, and having people who will do you favours (from not cutting you up on a road junction, to giving blood) is to do with your social circle, how much trust there is with the people you interact with, and how involved you are with society. High social capital is good.

Problem being, social capital - in America at least - has been in sharp decline since the early 1960s, and people are worse off because of it. The roots of this decline are in the early 1940s, and more in the post-war period. Putnam points the finger at commuting, suburbanisation and television as causes for this drop. However, what Jared Diamond's Guns, Germs and Steel teaches us is to look beyond proximate causes and search for the ultimate factors.

Sure, says Diamond, the Europeans were the ones who invaded South America and not vice-versa, and the proximate cause is that the Europeans had guns, ships, civilisation. But ultimately, why was that? It was because there were domesticable crops and animals in Euroasia, and barely any across the Pacific. It was because the crops there were spread easily on the long east/west axis of the European landmass, but America has a north/south axis which means a climate change gradient that crops can't move along so easily. Such are ultimate causes.

Aren't we already discovering the ultimate causes of social capital decline? Possibly, but I doubt it. If were weren't primed for a collapse, wouldn't that generation that grew up with television internalise it in a different way? Would centralised television networks have arisen? If the pursuit of social capital is intrinsic to human nature, wouldn't those values override the damage to the community that suburbanisation causes, and create a different kind of not-rural-but-not-urban lifestyle?

My contention: that humanity, being part of the real world, is maximally complex. Social capital isn't just something which is accidentally high or low depending on the structure of our society -- if it's something useful to humanity, then the feedback loop must be closed: humanity will have evolved in such a way as to promote social capital.

And because humanity's been around for a long time, there'll be a stabilising influence, a Le Chatelier's principle to keep the equilibrium at a point of high social capital. The human race must be self-selecting for just this attribute.

Well so we're obviously no longer in equilibrium. What's happened? Look at the dates -- the roots of the decline are around 1945. Marie Curie started fractioning radioactive compounds around the turn of the century, and atmospheric radiation started rising in the early 1940s till 1960s with nuclear tests in Nevada and elsewhere, and has been rising since. The cumulative effect of this means that people born after 1945 can be distinguished by their radioactivity from people born before that year. As people grow up with radioactivity, or later are born to radioactive parents, the effect grows. Global fertility has halved since 1945.

Putnam has noticed that social capital took another beating in the 1990s -- is this to do with the growth of materialism, or the Chernobyl accident of 1986?

Here's a thought. Whenever you make a decision about what to do, you weigh up the costs and benefits of generalised reciprocity. Basically speaking, if you're altruistic, what fraction of that altruism will eventually return to you from people in general? When it comes around, how much is gone? That estimated factor is how you measure whether to be a good citizen. It's an behavioural influence universal to all our activities just like mood, or tiredness. And since feeling, anxiety and so on are governed by brain chemistry, why not the reciprocity factor?

Perhaps, just perhaps, there's a chemical in the brain which sets, by its level, your expectations of society. And the processes that produce that chemical are especially susceptible to the unprecedented levels of radiation now present. And as radioactive substances build up in mothers, and move up the food chain and accumulate, so the proportion of affected people grows in society, and the effects of this diminished social capital makes themselves felt.

It all fits. Here I quote from an analysis of current-day social capital, and a study that measures how much social capital is in present in the states of America, Social Capital Measurement and Consequences: "There are a few outliers to the general pattern. Nevada is lower than where it should be; perhaps if you know something about Nevada you have guesses as to why this might be true". Indeed.