There's an account here of an experimental exercise comparing the brain activity of subjects who give honest responses about what they foresee the result of a coin toss to be and subjects who lie. The researchers findings are as follows:
Greene and Paxton had hypothesized that if deciding to be honest is a conscious process - the result of resisting temptation - the areas of the brain associated with self-control and critical thinking would light up when subjects told the truth. If it is automatic, those areas would remain dark.
What they found is that honesty is an automatic process - but only for some people. Comparing scans from tests with and without the opportunity to cheat, the scientists found that for honest subjects, deciding to be honest took no extra brain activity. But for others, the dishonest group, both deciding to lie and deciding to tell the truth required extra activity in the areas of the brain associated with critical thinking and self-control.
We are told, as well, that the two researchers draw no conclusions from their experiment about how the honest and dishonest groups would behave in non-experimental conditions, so we can leave this issue aside. But, even doing so, I find the way the above-reported result is formulated questionable. Greene and Paxton, the article says, suggest that 'the assumption that deciding to lie or to tell the truth is within our conscious control' may be flawed. Telling the truth might rather be 'an automatic process'.
Naturally, honest people have a default mode when they speak; they tell the truth as a matter of course. As with other things they do regularly, they don't have to work through their reasons for doing it on each separate occasion. 'Should I pay my electricity bill, walk facing forward, drive on the right (i.e., correct) side of the road, look with both eyes, etc, and if so why?' But the claim - on this account anyway - that such actions aren't within our conscious control is a stretch, to put it no more strongly. Even if it takes an instant to adjust when seeing the need, say, for a tactful lie, we can do it just like that. The fact that some things take longer than others doesn't show that the things that take less time aren't chosen, and therefore under our control.