A professor has found a reason why some people find the smile of presidential candidate Sen. Ted Cruz, R-Tx., unappealing.

Richard Cytowic, a professor of neurology, attributed the unpleasantness of Cruz’s face to his unconventional smile in a Psychology Today article this week.

Crytowic, after noting how several members of the Republican Party have shown disdain for Cruz, said humans read faces and make instant judgements based off facial expressions, a concept that dates back to the survival instincts of stone-age humans.

“Senator Cruz’s countenance doesn’t shift the way I expect typical faces to move. Human faces can’t help but broadcast what we feel, what we may be thinking, and even what we may intend,” he wrote.

He wrote that Cruz’s mouth typically tightens into a straight line when he smiles and the corners of his mouth bend down, not upwards. Downturned expressions, he stated, usually signal disagreeableness or disgust, but in a natural smile the corners of the mouth go up and are controlled by voluntary muscles.

“For the record I am not a Democrat. I’m at a loss to verbalize what unsettles me so when I watch the freshman senator,” Crytowic wrote. “But it leaves me cold.”