A longtime friend is bipolar and she has lied frequently -- I'd almost say consistently -- in the 20+ years we've known each other. She lies about things large and small. She even lies when the chances of getting caught are nearly 100 percent.
(If she weren't bipolar I'd drop this friendship, but I realize she has a major mental illness and needs the support of friends; besides that she's marvelous company much of the time.)
I've wondered why she lies and frankly I don't have a scientific answer for it. But I have some theories.
1. She habitually doesn't think of consequences. So in the moment I don't think she sees there's a down side to lying.
2. She's often in a state of mild mania (hypomania) and in these times she's not very grounded in reality. So it's hard to tell if what I'm hearing is a lie or fantasy on her part. (It's often not easy to tell when she's hypomanic vs. just happy and having a very good day.)
3. A symptom of mania is out-of-control ambition/self esteem. When manic, my friend thinks she can become a corporation president, start a company, become a community star, whatever. So during those times she'll say things that are blatant lies but feed into her need to feel special -- or blameless.
4. Unmedicated bipolars simply don't think the way normally functioning people do. My friend is often unmedicated. So she'll say whatever she thinks works to make people think she's the star of the show. If it's a lie, that seems to be OK. I don't sense she thinks there's anything wrong with that. That's not the way normally functioning people think, but like I said, unmedicated bipolars aren't normal.
I've called her -- gently -- over the years on her lies. But it hasn't made any dent in this habit. Rather she'll defend her lie or try to explain it away or give a weak "I'm sorry."
I don't think, at least with my friend, that there's any way to get her to stop lying. All one can do is be aware this is a pattern and take the necessary precautions. For example, I might give her money if she needed it. But I'd never loan her money on her word that she'd repay it.
Experience has taught me better.