Time to confess my guilty maternal secret: I am a sneaky snoop. I’m so desperate to know what is going on in my teens’ lives that I resort to reading things I should not, looking into drawers I’ve no business to look into and craftily glancing at open computer screens or unlocked mobile phones.

When they were little, it wasn’t hard to keep them safe. I easily uncovered misdemeanours due to their tendency to blurt out confessions in the manner of a gloating Bond villain. But once children hit adolescence, the secrets’ safe gets locked.

In the world of perfect parenting, our kids would obviously feel they could tell us anything. In reality, much of their new, almost-adult life remains hidden. There are no health-and-safety rules you can apply — and not knowing what’s going on and who’s involved scares the hell out of me.

This week, my 14-year-old was unusually sad. She wouldn’t say why, so I did what any sneaky snoop would do and I tidied her room while she was out.

I read something I should not have (not a diary — I would never do that) and by a process of emotional elimination worked out what was upsetting her. When she got home, I tackled the subject casually as if I’d developed a new-found telepathy and we put the situation right. She was happier, all the gloom and dread lifted. But were my actions right? It troubled me, so I ventured into some online chatrooms to see what other parents did. As long as they are under your roof, the majority of them said, then I had every right to keep everyone safe by whatever means I deemed necessary.

Then I turned to an expert. Nancy Darling is a professor of psychology in the US who has spent 25 years studying teens. In her opinion, snooping is always wrong. Intrusive parenting — where you overstep “normal” privacy boundaries — is a disaster because it makes teens ever more secretive. Instead, you must wait for them to come to you.

Teens have a strong sense of conscience, they want their independence and will respond better if you offer them it in return for trust. But break the trust once and you’re doomed.

In psychological terms, invading privacy denies your child their sense of “integral self”. You are striking at the very core of who they are (there is neurological science to back this up), so you must not do it.

It’s only acceptable to overstep normal boundaries when you have knowledge of immediate and severe harm, or if the child has previously broken your trust seriously. In my case, this means that observing changes in my children’s mood or behaviour, and gentle inquiry, is as far as my snooping is now allowed to go.