In 1922, in “Tractatus Logico-Philosophicus,” the philosopher Ludwig Wittgenstein wrote, “The limits of my language mean the limits of my world.” The words that we have at our disposal affect what we see—and the more words there are, the better our perception. When we learn to speak a different language, we learn to see a bigger world.

Many modern language researchers agree with that premise. Not only does speaking multiple languages help us to communicate but bilingualism (or multilingualism) may actually confer distinct advantages to the developing brain. Because a bilingual child switches between languages, the theory goes, she develops enhanced executive control, or the ability to effectively manage what are called higher cognitive processes, such as problem-solving, memory, and thought. She becomes better able to inhibit some responses, promote others, and generally emerges with a more flexible and agile mind. It’s a phenomenon that researchers call the bilingual advantage.

For the first half of the twentieth century, researchers actually thought that bilingualism put a child at a _dis_advantage, something that hurt her I.Q. and verbal development. But, in recent years, the notion of a bilingual advantage has emerged from research to the contrary, research that has seemed both far-reaching and compelling, much of it coming from the careful work of the psychologist Ellen Bialystok. For many tasks, including ones that involve working memory, bilingual speakers seem to have an edge. In a 2012 review of the evidence, Bialystok showed that bilinguals did indeed show enhanced executive control, a quality that has been linked, among other things, to better academic performance. And when it comes to qualities like sustained attention and switching between tasks effectively, bilinguals often come out ahead. It seems fairly evident then that, given a choice, you should raise your child to speak more than one language. Indeed, papers touting “Creativity and Bilingualism,” “Cognitive Advantages of Bilingual Five-Year-Olds,” “A Bilingual Advantage in Task-Switching,” “Bilingualism Reduces Native-Language Interference During Novel-Word Learning,” and “Good Language-Switchers Are Good Task-Switchers”—and the resulting books with provocative titles such as “The Bilingual Edge” and “Bilingual Is Better”—suggest that raising a bilingual child is, in large part, a recipe for raising a successful child.

From the age of eleven, Angela de Bruin spoke two languages. Born in the nineteen-eighties in Nijmegen, a small town in the Netherlands, de Bruin spoke Dutch at home, and, in school, immersed herself in English. She became fascinated by bilinguals, and read avidly about the cognitive advantages that being fluent in more than one language was supposed to provide. In college, she took up linguistics and neuroscience. And, in 2012, de Bruin enrolled in the psychology graduate program at the University of Edinburgh to further pursue the link between bilingualism and cognition.

She came to the program fully expecting to study the extent to which her bilingual brain was adapted to succeed. “I had the impression that there’s a really strong effect of bilingualism on executive function,” de Bruin told me recently. Then, she carried out her first study. Normally, to test for an edge in executive function, you give a version of a task where people have to ignore certain stimuli while selectively focussing on others. For instance, in the commonly used Simon task, you are shown pictures (often arrows) on either the left or right side of a screen. If you see a right-pointing arrow, you press the right key. It doesn’t matter on which side of the screen the arrow appears; the only thing that matters is the direction in which it points. Typically, people have faster reaction times on congruent trials—when the right-pointing arrow actually appears on the right, and vice-versa. Bilinguals are supposed to have an advantage in the incongruent trials: when the left arrow appears on the right, and the right arrow appears on the left.

When de Bruin looked at the data, though, in three of the four tasks testing inhibitory control, including the Simon task, the advantage wasn’t there. Monolinguals and bilinguals had performed identically. “We thought, Maybe the existing literature is not a full, reliable picture of this field,” she said. So, she decided to test it further.

Systematically, de Bruin combed through conference abstracts from a hundred and sixty-nine conferences, between 1999 and 2012, that had to do with bilingualism and executive control. The rationale was straightforward: conferences are places where people present in-progress research. They report on studies that they are running, initial results, initial thoughts. If there were a systematic bias in the field against reporting negative results—that is, results that show no effects of bilingualism—then there should be many more findings of that sort presented at conferences than actually become published.

That’s precisely what de Bruin found. At conferences, about half the presented results provided either complete or partial support for the bilingual advantage on certain tasks, while half provided partial or complete refutation. When it came to the publications that appeared after the preliminary presentation, though, the split was decidedly different. Sixty-eight per cent of the studies that demonstrated a bilingual advantage found a home in a scientific journal, compared to just twenty-nine per cent of those that found either no difference or a monolingual edge. “Our overview,” de Bruin concluded, “shows that there is a distorted image of the actual study outcomes on bilingualism, with researchers (and media) believing that the positive effect of bilingualism on nonlinguistic cognitive processes is strong and unchallenged.”

De Bruin isn’t refuting the notion that there are advantages to being bilingual: some studies that she reviewed really did show an edge. But the advantage is neither global nor pervasive, as often reported. After her meta-analysis was complete, de Bruin and her adviser ran an additional series of studies, which they have just submitted for publication, hoping to find where the limits of bilingual advantage lie, and what the real advantage may actually look like. To test for a possible boost, they examined three different groups (English monolinguals, active English-Gaelic bilinguals who spoke Gaelic at home, and passive English-Gaelic bilinguals who no longer used Gaelic regularly). They had each group take part in four tasks—the Simon task, a task of everyday attention (you hear different tones and must count the number of low ones while filtering out the high ones), the Tower of London (you solve a problem by moving discs around on a series of sticks to match a picture of what the final tower looks like), and a simple task-switching paradigm (you see circles and squares that are either red or blue, and must pay attention to either one color or one shape, depending on the part of the trial).

In the first three tasks, they found no difference between the groups. On the last, they thought they’d finally detected an advantage: on the switch trials—the trials immediately after a change from shape to color or color to shape—the bilinguals, both active and passive, seemed to be quicker. But when the researchers dug deeper, they found that it wasn’t so much a case of switching faster as it was being slower at the non-switch trials, where shape followed shape and color followed color.

So does that mean that there’s no such thing as a bilingual advantage? No. It’s just one study. But it adds further evidence to the argument that the bilingual advantage is sometimes overstated. “I’m definitely not saying there’s no bilingual advantage,” de Bruin says. But the advantage may be different from the way many researchers have described it: as a phenomenon that helps children to develop their ability to switch between tasks and, more broadly, enhances their executive-control functions. The true edge, de Bruin believes, may come far later, and in a form that has little to do with task-switching and executive control; it may, she says, be the result of simple learning.