There's a classic experiment in psychology.

Suppose you're watching a run-away railway car. It's big, it's heavy, it's going fast and out of control. A little way along the track splits, and the car might go right, or it might go left. On the right track a man is working, wearing hearing protection. There's no way you can warn him in time. On the left track a car is stalled with a family inside, mom, dad, two kids. They don't notice the coming disaster, either.

You can't know which way the run-away train is going to go. But which track would you prefer it to go on?

Suppose there's a switch by your hand. You can't tell which way the train is going to go, but if you press the switch, then the train will definitely go right. One man will be killed, but the family will survive.

Whatever choice you might make, most people will choose not to press the switch. They prefer the 50/50 chance of killing an entire family by not doing anything to the certain chance of saving their lives by doing something that will kill a man.

The reason for this is buried in our processing of moral choices. Moral thinking is something we all do instinctively, and in most cases we do it well enough. But under certain circumstances our moral judgment seizes up, like some kind of rickety steam engine that needs a bolt tightened. This is an excellent example of such an instance.

It is doubtful that any of us will ever be in the position that is described by that psychology experiment, right?

Wrong.

Right now tens of millions of parents in America face exactly that kind of a choice. An H1N1 flu epidemic is threatening this country. It is already hospitalizing and killing people around the world. The risk to our children is significant: at least 1 in fifteen of non-immunized children will be hospitalized due to the H1N1 flu. If the epidemic gets bad enough, then the overburdened health care system may well not be able to prevent many deaths.

But there is a certain risk to the vaccine, as well. The risk is tiny for most serious problems, on the order of 1/100,000 or less.

If parents do nothing, they risk a 1 in fifteen chance that their children will become seriously ill. If they get their children immunized, they risk a much lower, less than 1/100,000 chance that their children will become seriously ill.

And yet parents around the country are deciding not to immunize.

They're deciding not to press that switch, gambling a family's life.

In actual fact, these parents are doing worse than gambling their own family's life. Since non-immunized children will get ill and spread the disease, parents who do not immunize their own children are gambling with the lives of everyone else, as well.

But because of the rickety old steam engine that runs our brains, it is hard for them to recognize the immorality of this choice.

Factcheck.org has more detailed information.