I see the direction the data is pointing.

The true test of one's fidelity to the scientific method is how you respond when the data tells you something different from what you believe to be true. I grew up internationally and have lived and worked internationally a good chunk of my adult life. Lived in seven countries on five continents and worked in dozens more. In that milieu, but especially growing up, most people I knew were bilingual and a significant portion multilingual.

I recall one global business meeting I attended with a close colleague. The executive presenting was discussing an analysis and the justification for a consequential but hotly disputed strategic decision. The executive was French, presenting in English, my colleague was Dutch, and the audience from all around the world. My buddy leaned over to me and whispered, "I am fluent in seven languages and that argument doesn't make sense in any of them."

Anyway, bilingualism was always a mark of intelligence and sophistication. There was a (usually) unstated assumption that monolingualism, especially of the Anglophone sort, was something of a scarlet letter. Returning to the States in my late teens though, I began to question that attitude. Much of the bilingualism and multilingualism I knew was borne of necessity. If you are Danish, you better learn German, English, or French or your horizons are limited. Was multilingualism really a mark of ability or simply a sign of necessity.

Despite rising skepticism, I had a residual sense that multilingualism was beneficial, almost as an exercise of the brain. The assumption was that to acquire a new language reflected a mental resiliency of a sort. I remained strongly inclined to believe that there was some benefit, difficult to measure as it might be.

I have seen much research over the years on the issue. The stronger the analytic model, the weaker were the demonstrated benefits of multilingualism. Still, there was variance in the models and there was no definitive resolution to the question of whether there are cognitive benefits to multilingualism.

Is bilingualism associated with enhanced executive functioning in adults? A meta-analytic review. by Minna Soveri Lehtonen, et al, makes it somewhat more difficult to sustain that confidence. From the abstract:

Because of enduring experience of managing two languages, bilinguals have been argued to develop superior executive functioning compared with monolinguals. Despite extensive investigation, there is, however, no consensus regarding the existence of such a bilingual advantage. Here we synthesized comparisons of bilinguals’ and monolinguals’ performance in six executive domains using 891 effect sizes from 152 studies on adults. We also included unpublished data, and considered the potential influence of a number of study-, task-, and participant-related variables. Before correcting estimates for observed publication bias, our analyses revealed a very small bilingual advantage for inhibition, shifting, and working memory, but not for monitoring or attention. No evidence for a bilingual advantage remained after correcting for bias. For verbal fluency, our analyses indicated a small bilingual disadvantage, possibly reflecting less exposure for each individual language when using two languages in a balanced manner. Moreover, moderator analyses did not support theoretical presuppositions concerning the bilingual advantage. We conclude that the available evidence does not provide systematic support for the widely held notion that bilingualism is associated with benefits in cognitive control functions in adults.

Well. What to say? Credible data tells me something different than I grew up believing. All I can do is contingently acknowledge that the best data available is either dispositive or at best ambivalent about any imputed cognitive benefit from bilingualism. I will still remain impressed by fluent multilingualists and accord them greater respect than the data suggests I should. Research on complex systems such as cognition and communication is rarely conclusive. But certainly I see the direction the data is pointing.