Out on the street, reading another person's face correctly can mean the difference between living and dying.
"What are their eyes saying?" asked William Stanley, 52, a North Philadelphia paper-company delivery man, who was homeless for a time in 1998. "Is there a mouth twitch? If the guy's smiling too much, bad things will be happening."
Stanley, who volunteers to help feed the homeless, believes that the people who live closer to the bottom can suss out another's character and intent more quickly than the folks who Uber around town or take their own Lexus to work. They have to.
Social scientists agree.
For lower-income people, success depends more on how much they can rely on others. For example, if you can't afford day care for your children, you have to depend on neighbors or relatives to watch your kids, said Kraus, who co-wrote a 2010 study on class differences in detecting emotion and continues to explore the topic. To learn who's dependable, you have to be able to decipher expressions.
In a lab, volunteers were asked to look at pictures of faces and describe which emotions were being displayed.
People with more education didn't do as well as those who didn't attend college.
In another study, college students from higher socioeconomic backgrounds had a harder time reading a stranger's emotions than did people from less fortunate circumstances.
"Other people's thoughts, intentions, or wishes loom larger in my outcomes if I'm lower income," Kraus said in an interview. "That's because, if something happens to me, I need to recruit other people to help me deal with situations."
As a person rises in economic standing, he or she doesn't need to be as acutely aware of others' emotions, Kraus said. Such people can afford to be more independent because they need other people less.
This can have larger implications, because people who make policy decisions in America tend to be of a higher economic status. "They're less connected to others' emotions, which could make for less compassionate policies," Kraus said.
He suggested that someone like President Trump, born rich, might have a hard time studying people to determine what they might need.
"I'd guess the president is lousy at reading emotions, because he doesn't have to," Kraus said.
Temple University sociologist Judith Levine found merit in Kraus's work.
"Upper-class people can afford to be oblivious to others," she said. "They're more focused on where they have to get to next, and not paying attention to what's happening around them."
Psychologist Ruben Gur, who teaches at the Perelman School of Medicine at the University of Pennsylvania, reviewed Kraus' work as well as a 2016 study by New York University researchers who found that wealthy people pay less attention to passersby than do people of lesser means.
Gur found that both studies "make sense" and are "hard to ignore," although he cautioned that it would be inaccurate to conclude that simply because a person is in the upper class, he or she cannot read faces. He added that there was just a small difference between how rich and poor reacted in the studies.
The NYU study – conducted by examining what people looked at as they walked New York streets wearing Google Glass electronic eyewear — concluded that people from privileged backgrounds are less likely to see people as rewarding, threatening, or otherwise worth paying attention to.
Rich people can afford to live separate lives, which limits their experiences with others, other researchers have said.
The Rev. Adam Bruckner, who feeds the homeless on the Benjamin Franklin Parkway and works with Stanley to serve others, said that unlike the well-off, people who have struggled live very social lives. "You can't be self-sufficient if you often need someone to help you get food and clothing," he said.
And, Bruckner added, the poor must deal often with scam artists who prey on people like them.
Bruckner praised Stanley as a man who's great at reading people. "He's spot on," Bruckner said. Stanley has protected volunteers who help the homeless from attack just by looking at someone and knowing something was off.
"No joke, this guy tackled a man who pulled a knife," Bruckner said. "He's a bodyguard for my volunteers."
Stanley doesn't boast, but the man who became streetwise after he was shot as a youngster in the Frankford projects is not too shy to say he can handle himself.
"If a person is too wide-eyed, or blinks too much, he is a problem," Stanley said, continuing his street tutorial. "It's something I can't teach. I can just size a person up.