El Kaliouby is training machines to master the fuzzy realm of human feelings. The Egyptian-American entrepreneur cofounded Affectiva, a venture-backed “Emotion A.I.” startup, as a research scientist at MIT in 2009. Since then, the firm has analyzed 6.8 million faces—the better for decoding how to tell a smile from a smirk—and signed up 25% of the world’s biggest companies as users. Marketers, eager to test ads and products, have pounced on the mind-reading technology, though it’s also being applied to more virtuous use cases, like assisting individuals with autism and diagnosing depression.