While childhood lying can be occasional, compulsive lying in children who have grown up is neither normal nor healthy. The great child psychoanalyst Donald Winnicott once described the emergence of lying in children as a developmental achievement. When children learn to lie, they make themselves partially opaque to parental scrutiny, creating a precious area of inner privacy in the process. Vital capacities like imagination and creativity require an inner sanctuary to grow in. Repeated lying damages trust, promoting anxious uncertainty instead. Psychoanalysis suggests that fear and enjoyment, either separately or in concert, drive antisocial behaviours like lying.
Look for the hurt beneath the lie. Winnicott argues that behaviours like lying or stealing begin innocently as attempts to cure an original "deprivation," which was anything the child experienced as a withdrawal of parental love. Most of the time, parents help their children recover from these setbacks, which might happen when, say, a mother has suffered a bereavement or a father becomes temporarily emotionally unavailable due to stressful life events. Mend the hurt early and the child regains faith in the world. But if it remains unnoticed, it functions as an unhealed emotional wound, a hole in love.
Redescribe the lie as a hope --- a hope that someone will find and mend the hole. Every time a child lies, she tries to show how the hole affects her. It drives her on a mission she cannot comprehend: she can't say "why" she lied because the "hole" doesn't exist as a conscious memory. It persists as a hidden experience, like a hungry baby demanding love. In infancy, good parents give love and care as rights, not favours; the hole belongs to this level of development and, unhealed, later drives children to take what they needs from the environment.
Explain that you understand that lying functions as a protective shield --- beneath it hides the unhealed hurt. Provided a parent notices early enough that pain and distress lie behind these antisocial tendencies, hope of full recovery remains. (Winnicott recommends "therapeutic spoiling" -- infilling the emotional hole with additional love and attention until the lying fades away.) But when "secondary gain" sets in -- the thrill and enjoyment that comes from lying and stealing -- things get much more difficult. Adult children almost invariably fall into this category. The unhealed "hole" means that they fear the world will never provide them with what they need (so they must take it or lie for it) and they obtain an addictive "kick" out of lying, too.
Keep a short journal. Discreetly, make notes in a pocket jot-pad whenever your adult child tells you something "fishy." Liars get away with lying largely because of everyone else's poor memory. Use the notepad as your auxiliary memory for tracking inconsistencies. Be patient, though --- the elements of the story that don't add up will inevitably emerge over time. Take care not to act punitively or humiliate your child when you decide to confront him with your suspicions -- if you do, antisocial behaviour gets reinforced because you just confirmed an entrenched belief that the world is not safe.
Be open but kind: "You're telling lie after lie because you mistrust the world. We don't want you feeling unsafe here with us. We won't condemn you or judge you and we won't shame you; we just want you feeling safe enough for honesty. But we know it'll take time and practice. That's why we're telling you whenever we think you're lying to us." Remain calm and warm throughout and resist expressing frustration or exasperation.
Don't just act kind -- find real kindness and compassion in your heart to guide you. Compulsive lying is ingrained pathology, overriding choice and good intentions. Compassion makes receiving lies easier to bear, not less. Your adult child needs lots of practice -- and forgiveness -- to ditch the habit. But liars need firmness, too; stay calm, warm and straightforward when you draw attention to a lie but don't dodge drawing attention.
Be prepared. You must remain committed to this approach and see it through: your adult child will almost certainly test you to see if you can survive the full impact of the underlying deprivation. If you give up, the antisocial tendency wins.