Meh. Maybe speaking two languages fluently doesn't have the advantage that psychologists are trying to measure. But it seems to me almost absurd to assume that there are no advantages. They might not justify the effort of learning a second language, though. But anyone who knows two or more languages has at least a better grasp of semantics as two meanings that are mapping to the same word in one language are often different words in another. For example: "sky" and "heaven" are the same word in German, but obviously different concepts that have their own words in English. Likewise, the noun "lead" has many meanings that map to different words in most languages.