Disability advocates were seeing red after two elderly women with medical conditions were allegedly strip-searched by TSA agents at New York’s JFK airport last December. You’d have to have a pretty thick skin not to empathize with an elderly, wheelchair-bound woman having her colostomy bag frisked. But the notion of one passenger being an unlikely terrorist also belies a discomfiting flipside: another passenger being a more likely candidate.

For the last few decades, social scientists have been teasing out the mental and physiological systems involved in profiling and social bias. Taken at face value, the biases look like simple prejudice, like assuming that black people are criminals, or that people from the Middle East are terrorists. But research on social cognition is revealing much more subtle and unconscious mechanisms behind these social biases. Case in point: objects can ‘grab’ properties from nearby objects, in a phenomenon scientists call illusory conjunction. For instance, a red circle next to a white triangle might make the triangle seem red. This same effect can also apply to social targets: a neutral face can ‘grab’ the emotion of the angry person next to it, causing the neutral person to be remembered as angry.

 In a recent paper published in the Journal of Experimental Social Psychology, researchers at Arizona State demonstrated that male faces are more likely than female faces to “grab” the anger from an adjacent face, while female faces are more likely to “grab” happiness. Using both photos of actual people and artificial images morphed to appear angry or happy, the scientists presented students with side-by-side images of faces (either male or female, happy or angry), along with two numbers they had to add in order to distract their conscious minds. After flashing the faces and numbers, a dot appeared on the left or right, and the students were asked to quickly recall the emotion of the face flashed on that side. The researchers were interested in the errors made by the students – particularly errors in which the emotion they thought they saw actually came from the other face.

Across two studies, lead author Rebecca Neel and her colleagues found that male faces were more likely to grab anger from the face next to them, and female faces were more likely to grab happiness. Interestingly, this was not just a matter of people seeing male faces as more angry and female faces as more happy (an effect shown in several previous studies): the errors were most common when the emotion came from the adjacent face.

While this grabbing seems like an automatic process of visual perception, it can be influenced by   biases we hold about the social world. Expectations shape perceptions (and misperceptions).  Since we tend to assume men are more aggressive and women are more nurturing, we’re more likely to see men as angry and women as happy. Snap judgments happen easily as we scan our environments looking for social cues. It may seem a serious design flaw in a diverse modern society, but our brains have evolved to make constant and routine approach-avoid assessments called ‘affordance management’ decisions. At the creature level, we’re trying to minimize risk and maximize opportunity. It’s more costly to miss cues of impending danger than it is to mistakenly assume that someone is threatening, so we err on the side of caution. Men have historically posed a greater physical threat than women, so the angry face is suddenly male. Females are seen as posing little threat and ample opportunity for connection, so our brains maximize opportunity by assuming a woman is friendly and eager to interact (or take care of us) even when she isn’t.

The top-down schematic expectations fuse with bottom-up illusory conjunctions from the early visual process, creating fertile ground for error (particularly when distracted or hurried).  This latest study adds to the prejudice literature by showing a new way that bias may creep into our lives: by perceptions of one person selectively "grabbing" certain stereotype-consistent emotions from the people around them. This suggests that when you're standing in the security line at the airport, it not only matters how you look, but who's standing next to you as well.