Lightman tells his ex-wife that whatever he finds out he will disclose both to her, the defense attorney, and the prosecution. That is not the way it usually works. Typically experts can only tell what they find out to those who hired them. If it isn’t useful or might help the opposing side the other side never find out, and neither does the jury.
Knowledge of how someone usually acts when being truthful — the baseline — is critical to spotting a lie. We each have our own behavioral style, our own mannerisms. If, for example, someone always talks hesitantly, then hesitant speech is his or her baseline. If such a person were to be hesitant when asked about a crime it doesn’t mean anything. But if it were not the person’s baseline then hesitant speech would raise the possibility of deceit. I usually ask people to tell me about the best and worst experience they had in the last month to discover their usual behavioral repertoire. I don’t ask them to lie, as Dr. Lightman did here, because unless the stakes for being caught are very high, as they would be if convicted of a serious crime, success in lying wont matter enough to arouse the emotions that might betray the liar.
When Lightman goes into his daughter Emily’s bureau drawer he is invading her privacy. As a parent I know how hard it is to resist the fear that your child may be engaging in dangerous behavior. But invading their privacy will likely teach them to be better liars. I believe it is only justified if the danger is very severe, very likely, and very imminent. It isn’t here.