In 1884, at his specially built Anthropometric Laboratory in London, Sir Francis Galton charged visitors three pence to undergo simple tests to measure their height, weight, keenness of sight and “swiftness of blow with fist.” The laboratory, later moved to the South Kensington Museum, proved immensely popular—“its door was thronged by applicants waiting patiently for their turn,” Galton said—ultimately collecting data on some 17,000 individuals.

One measure that deeply interested Galton, who is recognized as “the father of psychometrics” for his efforts to quantify people’s mental abilities (and scorned as the founder of the eugenics movement because of his theories about inheritance), was speed. He believed that reaction time was one proxy for human intelligence. With a pendulum-based apparatus for timing a subject’s response to the sight of a disc of paper or the sound of a hammer, Galton collected reaction speeds averaging around 185 milliseconds, split seconds that would become notorious in the social sciences.

For decades other researchers pursued Galton’s basic idea—speed equals smarts. While many recent tests have found no consistent relationship, some have demonstrated a weak but unmistakable correlation between short reaction times and high scores on intelligence tests. If there is a logic to the link, it’s that the faster nerve signals travel from your eyes to the brain and to the circuits that trigger your motor neurons, the faster your brain processes information it receives, and the sharper your intellect.

Psychologist Michael Woodley of Umea University in Sweden and his colleagues had enough confidence in the link, in fact, to use more than a century of data on reaction times to compare our intellect with that of the Victorians. Their findings call into question our cherished belief that our fast-paced lives are a sign of our productivity, as well as our mental fitness. When the researchers reviewed reaction times from 14 studies conducted between the 1880s and 2004 (including Galton’s largely inconclusive data set), they found a troubling decline that, they calculated, would correspond to a loss of an average of 1.16 IQ points a decade. Doing the math, that makes us mentally inferior to our Victorian predecessors by about 13 IQ points.

The Victorian era was “marked by an explosion of creative genius,” Woodley and his colleagues write. There was, after all, the first world’s fair, the rise of railways, anesthetics and tennis. While environmental factors can surely boost specific skills (some researchers thank better education and nutrition for increases in IQ over the last few decades), Woodley appears to argue, from the biological perspective, our genes are making us dumber.

Critics, however, aren’t as quick to agree on our apparent downward mental trajectory. Whether or not we’re dumbing down, they argue, resurrecting old data from independent studies with different protocols is not the best way to find out. Reaction times are known to vary depending on how much a study emphasizes accuracy, whether participants practice in advance and the nature of the test signal itself. Some researchers now think that other measures of reaction times are more telling. They look at the variability in response time rather than the average, or they add decision making, so you react to a flash of light only if it is, say, red.

As a society we certainly equate speed with smarts. Think fast. Are you quick-witted? A quick study? A whiz kid? Even Merriam-Webster bluntly informs us that slowness is “the quality of lacking intelligence or quickness of mind.” But we also recognize something counterintuitive about accepting full-stop that people who react faster are smarter. That’s why, even though athletic training improves reaction time, we wouldn’t scout for the next Einstein at a basketball game. Intelligence probably has a lot to do with making fast connections, but it surely has just as much to do with making the right connections.

Even the perception of speed can be deceptive. When things come easily or quickly, when we don’t have to struggle, we tend to feel smarter, a concept termed fluency. In one study, Adam Alter and fellow psychologists at New York University asked volunteers to answer a series of questions typed in either a crisp, clear font (a fluent experience) or a slightly blurred, harder to read version (a disfluent one). The people who had to work harder ended up processing the text more deeply and responding to the questions more accurately.

We tell athletes to think fast. But when we want a well-reasoned decision, we say think long and hard, which isn’t all that different from think slow.