Self-comforting gestures reassure an ill at ease person. Other reasons why a person might be uncomfortable would have to be ruled out before interpreting this movement as evidence of lying.
Kids Lie to Parents?
Like Lightman, I rarely challenged my daughter when I knew she was lying. Foster is right; everyone is entitled to privacy; to keep some secrets.
Later in the show Foster tells Emily that Lightman knew she was lying on the phone, but he also knew he shouldn’t challenge her every time she tries to conceal something from him. The show closes with Emily telling Lightman he doesn’t catch all her lies.
My daughter still believes that I didn’t either; that’s because I, too, didn’t challenge her every time she lied. Now, at 28 there rarely is a reason for her to lie to me.