In the United States, there is a long association between putting your hand over your heart and affirming something sincerely. Most notably, when people in the United States say the Pledge of Allegiance or sing the national anthem, they are encouraged to salute the flag either by putting their right hand over their heart.
It appears that placing one’s hand over one’s heart as part of a ritualized oath of allegiance goes back in the United States at least to 1870; and that placing one’s hand over one’s heart to affirm that what one says is true goes back in the United States to at least 1821.
Something as simple as placing our hand over our heart, can trigger us to behave more morally. But at the same time, skilled liars could use this simple cue to manipulate others into believing that what they say is the hand-over-their-heart truth.