This morning’s Observer column.

In 1936, a Harvard sociologist called Robert K Merton published an article that has haunted politicians and corporate strategists ever since. It was entitled “The Unanticipated Consequences of Purposive Social Action” and set out what has become known as the Law of Unintended Consequences (of which Murphy’s Law is a special case).

Merton’s Law says that any purposeful action will produce outcomes that its proponents did not anticipate…