Some years ago a colleague of mine bought a diesel car because she believed that diesel fuel was less polluting than petrol.
You could travel further for a gallon than using petrol and this too, she thought, must be good for the environment. And the government encouraged this with a lower tax on diesel. She made her colleagues feel bad because we all had cars with petrol engines.
Now we discover that diesel has been a major contributor to climate change. An unintended consequence.
What is true in everyday life is true in everyday politics. There are always unintended consequences.
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So, we voted to leave the European Union. Who would have thought that an unintended consequence might be the break-up of the United Kingdom as the Scottish parliament seeks another referendum to win independence in order to keep Scotland in the European Union?
We always need, therefore, to be careful that in dealing with one problem we don’t produce another.
This is as true in tackling crime as anything else. Take the use of tasers.
We have been rightly shocked in recent years at the increase in violence offered to police officers by some very aggressive people, not least as they have tried to resist arrest.
The natural instinct is to say that all officers must be routinely equipped with a taser. We are right to be concerned with the safety of police officers.
We want to reduce violence towards them and calm potential confrontations. Tasers may be an answer or part of the answer.
But there could be unintended consequences and we need to think it through.
For instance, a recent study by Cambridge University found that City of London police armed with tasers faced twice as many assaults as unarmed colleagues.
The researchers thought this was because when people saw the tasers they immediately started to become more aggressive. Far from calming a situation it can make it worse.
Police armed in this way look intimidating – more like a police force that is imposing itself on communities rather than acting with their consent.
Tasers have their place, but we need to be careful how they are deployed, which is why this has to be an operational decision kept under constant review.
What in the end keeps our police safe is that however they act, they do so with public opinion one hundred percent behind them.