Why put your hand/ fist on chest/ heart when talking to a person? (mostly girls)

I have goggled this, couldn't find anything. I have an interest in the psychological aspects of body language and figured some one here must have an idea. When I was talking to a girl she put her fist on her chest when she said something. The tone in her voice sounded more serious at that point as well. I have noticed this with other girls too, I want to know if any one else has noticed this? Or is there any possible meaning to it? I believe the person is being sincere about what they are saying while the hand is there.
Why put your hand/ fist on chest/ heart when talking to a person? (mostly girls)
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