There's more and more evidence that being bilingual makes you smarter and keeps your brain functioning for longer. Here's a summary from the New York Times (but there's a huge amount of material on this topic out there, as a quick Google search will demonstrate):
This view of bilingualism is remarkably different from the understanding of bilingualism through much of the 20th century. Researchers, educators and policy makers long considered a second language to be an interference, cognitively speaking, that hindered a child’s academic and intellectual development.
They were not wrong about the interference: there is ample evidence that in a bilingual’s brain both language systems are active even when he is using only one language, thus creating situations in which one system obstructs the other. But this interference, researchers are finding out, isn’t so much a handicap as a blessing in disguise. It forces the brain to resolve internal conflict, giving the mind a workout that strengthens its cognitive muscles.
Bilingualism’s effects also extend into the twilight years. [...] Individuals with a higher degree of bilingualism — measured through a comparative evaluation of proficiency in each language — were more resistant than others to the onset of dementia and other symptoms of Alzheimer’s disease: the higher the degree of bilingualism, the later the age of onset.
It's also a well-known fact that Scotland punched well above its weight inside the United Kingdom. It appears to me that this started some decades after the 1707 Acts of Union and slowly started to fade out in the 20th century.
Interestingly, most successful Scots must have been bilingual (or even trilingual) during this time -- you needed to know English well to succeed, but in Scotland everybody spoke only Scots and/or Gaelic (depending on where they lived). I'm not sure when Standard Scottish English started replacing Scots and Gaelic as the primary language of large numbers of Scots, but it must have been a relatively recent event. For instance, when Norn was dying out around the time of the Acts of Union, it got replaced by Scots, not by English. On the other hand, the vast areas of the Highlands where Gaelic died out after the Clearances ended up speaking English, not Scots.
It's tempting to think that one of the factors that allowed Scotland to punch above it weight was the near-universal bilingualism. If this theory is correct, making all Scots bilingual again by supporting and promoting both Gaelic and Scots will make Scotland a more successful country in the future.