My 20-year-old daughter is a liar. Her pattern of lying has been consistently punished. She knows that she will get caught and punished. From a young age, she has been told that this is unacceptable behavior and the sin nature. She feels it is best to run the risk of getting caught. Her punishments have fit the crime. She seems to have no remorse for her action and the problems that it causes for the entire family. What should I do?
I'm in a difficult spot with your question. It is rare for a person to be able to look at their own situation and accurately pinpoint where the trouble lies. After all, if such happens, the problem would be corrected. Hence, I'm left to reading between the lines -- trying to guess what was overlooked. You claim you have done everything right, but have gotten the wrong results. Since I trust that God's methods do work, I must assume the situation is not precisely as you describe it.
No liar "knows she will get caught and punished." The whole motivation behind lying is that the lie will either avoid or delay an unwanted consequence. Liars gamble on the fact that sometimes they can get away with a lie. You might have managed to punish the lies you caught, but the fact that the lies continue indicates that there have been lies which she has gotten away with using. These "successes" are just frequent enough to keep her hopes up that she will get away with the next lie.
Since you state that this has been going on from a young age until her current age of twenty, the habit of lying has become well ingrained into her character. The Bible doesn't actually speak of "a sin nature." That particular phrasing is used in the New International Version. The literal wording is "flesh," such as in Romans 7:25, meaning following after the ways of the world. The interpreters of the NIV see this as meaning that people can't help sinning because they inherit sin from Adam. However, Ezekiel 18 clearly states that people sin because they choose to do so, not because it was forced upon them. Yet, when sin is repeatedly done, it can become ingrained. As you indicate, she has become calloused to the sin of lying, which is a deep misfortune. As Paul warned, there are people "speaking lies in hypocrisy, having their own conscience seared with a hot iron" (I Timothy 4:2). Tell lies long enough and it no longer bothers the liar.
Most compulsive liars are involved in addictive sins, most often drugs and alcohol. The only way to maintain their habit is to lie -- not just to others, but also to themselves. Like the evil men Paul warned about, they "grow worse and worse, deceiving and being deceived" (II Timothy 3:13). Drug users tell themselves lies such as, "I can quit anytime I want," or "I don't have to use it, I just want to use it," or "I'm just going to use it one more time, then I'll stop." They tell lies to others to cover up their tracks and to avoid facing reality. Who wants to admit that a chemical completely controls his life? Even after a drug user stops, I have noticed that many have a hard time stopping the lies. I have dealt with several ex-drug users who have to consciously work at telling the truth. Lying is their first impulse even when a lie is unnecessary.
The past cannot be undone. We can only deal with the present. Since she persists in lying and that lying is causing the family problems, the best solution for the moment is to give her a huge dose of reality. She is twenty -- more than old enough to live on her own. Find her an affordable apartment. Pay the first month or two's rent and move her out. Tell her bluntly, "Honey, we love you, but your lies and bad habits are disrupting the family. You're an adult, so it's time you live like one. Dad and I have paid the first month's rent. After that, you are on your own. Utilities, insurance, college, food, and the like are now your responsibility as an adult. We hope you will straighten out your life one day, but until then call once in a while to let us know you're still alive."
What I'm advising is that you cut the apron strings and stop supporting her bad habits. It will be rough for her and seemingly near impossible for you. But she has made her choice. You are no longer in a position to rescue her. She will have to decide if being a Christian is worth changing her life to obtain it, and then she will have to implement those changes on her own. Give her sound advise when she asks, have her over for the holidays and family gatherings as the polite thing to do, but keep your pocketbook closed. You gave her a start. What she does with the rest of her life must be her choice and the result of her own actions.