Before you start

This is a short book and you can read it in two or three sittings. It's about one habit. You lock the door, walk to the car, and go back to push the handle. You turn the hob off and look at it again from the doorway. You send an email and open it in the sent folder to read it through, and then once more. Each check is meant to settle something, and each one leaves you a little less settled than before, so you check again.

I'm going to argue three things. The first is that checking makes sense from the inside. The things you're checking matter, a check costs seconds, and memory does fail. The second is that repeated checking does the opposite of what it's for. The research on this is unusually clear. Checking something again doesn't make you more accurate. It makes your memory of the first check dimmer and less detailed, and it's the dimness you feel as doubt. The doubt you're trying to remove is produced by the checking. The third is practical. You check once, slowly, with attention, and then you let the doubt arrive and pass without going back. Chapters 17 to 20 cover how.

This book doesn't say that checking is silly or that the things you check don't matter. A locked door and a cold hob matter. It says that the first check does the job and the rest undo it.

Three cautions.

If checking takes up a lot of your time, if you can't leave the house or get to sleep until rituals are complete, if the checking causes real distress or is getting in the way of work or relationships, that may be obsessive compulsive disorder. It's a common condition, it's nobody's fault, and it's treatable. The NHS says you can see a GP or refer yourself directly to an NHS talking therapies service. The ideas in this book come from the same research that the treatment is built on, but a book isn't treatment, and if that description fits you, please get the real thing. Chapter 21 says more.

If your checking is about a baby, such as repeatedly checking they're breathing, and it comes with unwanted, frightening thoughts, the NHS describes this too, and says to speak to a GP or health visitor.

And if you check because you've genuinely been leaving things, the hob on, the door open, more than once in recent months, that's worth mentioning to a GP, especially if it's new. The habit in this book is one where the checks keep finding everything fine.

The notes at the back give my sources and their limits. Where I'm giving my own reasoning and not a finding, I'll say so.