Are emotions a human universal? Charles Darwin suggested it (I doubt he was the first), at a time when biological/genetic determinism and pseudoscience were running rampant, usually under the guise of scientific racism. By the beginning of the twentieth century, the pendulum began to swing back. Anthropologists like Boas and Mead rejected biological determinism, and viewed culture as the prime determinant of the human experience. Emotions, to them, were cultural creations and not innate human behaviors (a stance as much political as evidence based).

That view held sway until the 1970s, when psychologist Paul Ekman conducted a series of simple studies in which he showed pictures of facial expressions to thousands of people across the globe. He found that six emotions—happiness, sadness, anger, fear, surprise, and disgust—were universal. People recognized them everywhere he asked, even in cultures where their expression was subject to rules of time, place, gender, and propriety—or even proscribed completely.

And so the pendulum swung back and most people began to view emotions as a shared innate human experience—albeit an experience that could be strongly regulated by cultural demands. (Ekman achieved such fame that the lead character of the series Lie to Me was based on him…and, oddly, given their ideological contretemps, Margaret Mead’s later work showed that in certain cultures being portrayed on network drama was a universally recognized social distinction)

Facial expressions, though, aren’t just static photographs like Ekman used. The dynamics of expressions are important, too—and results from one recent study might challenge how we think about what’s universal in emotion. In that study, researchers generated an animated sequence for each of Ekman’s six emotions, showing not just where our faces end up, but how they change as we express different emotions. They then used “fancy math” (highly technical term) to isolate which parts in each sequence—which aspects of emotional expressions—actually allow us to discriminate one emotional expression from another.

What they found is that timing matters. Whereas happiness and sadness share little in common and are easily distinguishable, other emotions aren’t. Fear and surprise, it turns out, begin similarly: the eyes widen and the jaws drop. Only later, when the eyebrows raise, are we able to distinguish fear from surprise. Anger and disgust are also similar: both start with a wrinkled nose and a funneled lip before we are able to distinguish them later. Thus, when an expression first forms, only four emotions are separable: happiness, sadness, anger/disgust, and fear/surprise.

Why might fear and surprise or anger and disgust be “linked” like this? The answer lies in the role of emotions as signals. Emotions aren’t just intrapersonal—they also communicate important information to people around us. Indeed, the social aspect of emotions is so important that we even empathetically feel emotions simply from seeing other people express them.

If we view emotions as signals, the authors of this study suggest, we might see that anger/disgust and fear/surprise are temporally related. Fear and surprise both mean that something is important right now; they carry connotations of urgency and fleeting (and fleeing). Anger and disgust, in contrast, are focused and longer-lasting; relevant but not urgent. So, the authors contend, these results may suggest the brain is really hard-wired to send and recognize just four universal emotional “signals”: happy, sad, be on your guard (anger/disgust), and alert! (fear/surprise).