Have you ever wondered why they have canned laughter on television? And why the laughter gets louder the less funny the programme?

It’s because, as tests show and TV producers of unfunny programmes understand, we laugh more when other people are laughing. We even commit suicide more when other people are doing it. No one likes to ask the first question, but once someone else does, almost every hand in the room is raised. We like to go with the flow, we humans do.

This habit can be a good one. It allows us to learn from each other. But it can also be a bad one. It can allow intelligent people to accept sloppy and inconsistent arguments as long as no one else questions them.

I only point this out because something has been puzzling me. In the past month there have been two big public arguments on basically the same topic. On both, the vast majority of sage people seem to agree with each other and the voices of dissent are relatively few. Yet the consensus in one argument is the exact opposite of the consensus on the other.

So, help me out here. Let me see whether I’ve got this right. We’ll start with marriage, shall we?

Iain Duncan Smith has established that there is an epidemic of family breakdown and that this is connected to poverty for many children. Some question the causality (Polly Toynbee states that “poverty causes separation”) but most agree that broken homes damage those brought up in them. This in turn is paid for by the rest of us — we pick up not only the welfare bill but the cost of increased antisocial behaviour. The data seems to show very clearly that cohabitation is associated with a far greater number of broken homes than are found when the parents marry. Most also agree that the problem is serious.

At the same time, there is a broad consensus that there isn’t much that can, or even ought, to be done about it. The political reason for inactivity is simple — there are too many unmarried parents with votes.

Even if the data may suggest that, on average, homes with two married parents are better for children, this is not remotely true of every home and not remotely possible in every circumstance. Voters think they know better than politicians what is good for them and their kids. They don’t like being told that their lifestyle choice may be damaging their children. And most commentators regard this attitude as reasonable. Politicians should respect the diversity of family life and keep their noses out of people’s bedrooms.

In any case, even if it were not politically damaging there isn’t much, everyone seems to agree, that government can really do about family breakdown. A few weedy tax breaks is hardly going to change things much. And using the bully pulpit of office to urge different behaviour is out of the question — too many politicians cohabit, are divorced or have affairs.

I think that just about sums it up, doesn’t it? I hope so, because now I want to move on to discuss the public consensus on obesity.

The starting point of the obesity consensus is almost identical to that on marriage. There is, apparently, an epidemic of fatness. The free choice of parents is damaging the welfare of their children and the rest of us are picking up the bill. It’s the same basic point — that society would be better if parents were more responsible.

At which point, consistency ceases. For the consensus on obesity is that we can, that we must do something about it. Now. Before we all catch it and die.

There may be a lot of fat voters, but they must be told to get with the programme. If those fat people and their fat children think they know better than politicians, well, then they are wrong. We have every right to interfere if they endanger their children’s health. This weight diversity cannot be allowed to continue; the State must intervene in people’s kitchens.

And there are things that can be done about obesity, we all agree. People are advertising chips on television, for heaven’s sake. It must be stopped. And perhaps we should tax unsaturated fat. Certainly we should be using the bully pulpit, making speeches about the evils of selling large bars of chocolate to unaccompanied minors and spending government money on adverts exposing the lethal properties of table salt.

What explains this discrepancy? It’s certainly not the strength of the evidence. Even the head of Ofcom, the body imposing the ban on fast-food ads, acknowledges that the influence of television advertising on diet is pretty weak. And that’s assuming that what is causing obesity is excess consumption of fast food. The data on this is far from conclusive.

The fast-food link is still a hypothesis. Lack of exercise seems likely to prove a stronger cause of obesity. Indeed, I wonder if the way that the fast-food ad ban will work is by removing the funding from television, making it so bad that all the children go out to play.

Meanwhile, the evidence from the United States is clear — welfare reform aimed at encouraging two-parent families and marriage education programmes both work. And it is common sense that it is a bad idea for us all to regard cohabitation and marriage as much of a muchness, when they clearly aren’t. Do we seriously think this has no impact on family breakdown? We can do something about marriage rates, of course we can.

So if it isn’t the strength of the evidence, what is it? It is will.

The reason why obesity is considered a problem that government can address and marriage isn’t is that with obesity there is someone else to blame. We can pretend that fat people and their children are simply victims. The culprits are Big Food, hiding round corners dealing out milkshakes to vulnerable children. We can pretend that some big anonymous corporation is at fault, rather than our friends or ourselves. What nonsense and what cowardice.

If all the obesity campaigners and their fellow travellers expended the same energy on the campaign for marriage and families as they spent on their crusade against salt and vinegar crisps, the world would be a better place. But if they can’t bring themselves to do that, then they should just leave us all alone and have done with it.

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