Suspicious minds are driven by testosterone, a study has found.
People are less likely to be trusting if they were exposed to high levels of the male hormone in the womb.
But they are not just good at spotting con-artists and cheaters, as was previously thought.
They are universally more suspicious of everyone, no matter how honest they appear, according to the research.
Scientists assessed pre-birth exposure to testosterone by looking at the length of ring and index fingers in a group of female volunteers. Although testosterone is a male hormone, it also circulates in women.
The findings, by a French team led by Dr Jean-Francois Bonnefon are published in the Royal Society journal Biology Letters.