In 1975, the psychologists Stephen West and T. Jan Brown conducted an investigation into the factors that made people more likely to help a stranger. What made their study unique is that they conducted the experiment twice, using two different methods.

In the first study, they staged a crisis. Sixty men walking on a college campus were stopped by a woman who made the following request:

“Excuse me, I was working with a rat for a laboratory class and it bit me. Rats carry so many germs – I need to get a tetanus shot right away but I don’t have any money with me. So I’m trying to collect $1.75 to pay for the shot.”

In some conditions, the woman held her hand as if it had been bitten; in other conditions, her fist was wrapped in gauze that had been soaked in artificial blood. Sometimes she wore an “attractive pant outfit and was tastefully made up” and sometimes she wore a blonde wig, white face powder and dark lipstick, “all of which were inappropriate for her natural complexion.”

Not surprisingly, men offered the most help when the woman was attractive and in urgent need of help, giving her an average of 43 cents. (Every man stopped to help in this condition.) In contrast, an “unattractive” woman with a bloody bandage received 26.5 cents on average, and only 80 percent of men offered help. The less severe conditions led to even less assistance: the men donated approximately 13.5 cents with two-thirds providing some amount of money.

So far, so obvious: when deciding whether or not to help a stranger, the most important variable is the severity of the situation. We might stop for a head-on collision, but not for a fender bender. If you're asking for money, it’s better to be good-looking.

But the most intriguing part of the paper came when the scientists tried to replicate their field study in a lab. Instead of faking an emergency on the street, the sixty male subjects were read a description of the injury (severe/not severe) and shown a photograph of the woman (attractive/unattractive.) Then, the men were asked how much money they would be willing to give her.

In this “interpersonal simulation,” the men were very generous. Interestingly, they gave the woman the most money in the unattractive/severe condition, offering her an average of $1.20, or four and a half times what their peers offered in real life. The same basic pattern persisted across every situation, with the men giving her far larger sums when she was a hypothetical. The lab subjects also insisted they wouldn’t be swayed by her appearance - they said they'd give more when she was less attractive - even though the field test strongly suggested otherwise. West and Brown conclude their 1975 paper with a warning: “The comparison of the results of the field experiment and the interpersonal simulation raise serious questions concerning the validity of the latter approach as a strategy for investigating human social behavior.”

I first learned about this study from a fascinating critique of modern psychology, published in 2007 by the psychologists Roy Baumeister at Florida State University, Kathleen Vohs at the Carlson School of Management, University of Minnesota and David Funder at the University of California, Riverside. In “Psychology as the Science of Self-Reports and Finger Movements,” Baumeister, et al. hold up the results of the West/Brown study as an example of the unsettling discrepancy between what we think we’ll do and what we actually do. Because it turns out that such discrepancies are a recurring theme in the literature. For instance, Baumeister, et al. note that “affective forecasting studies” – research in which people are asked how they will feel if x happens – “systematically show the inaccuracies of people’s predictions” about their own future emotions. Meanwhile, financial decision-making research reveals that people are “moderately risk averse” when dealing with pretend money, but become far more risk averse when large amounts of real cash are involved. Other experiments show that merely asking people about their preferences can alter their preferences; the act of introspection has a distorting effect. As the psychologist Timothy Wilson famously argued, we are all “strangers to ourselves.”

And yet, despite this surplus of evidence, Baumeister and colleagues document a steady decline in the percentage of studies that actually look at behavior, and not just our predictions of it. Here’s the trendline of research published in the elite Journal of Personality and Social Psychology over the last forty years: