Why is learning a foreign language so darn hard?

I work in an toddler language lab, where we study small children who are breezing through the process of language acquisition. They don't go to class, use note cards or anything, yet they pick up English seemingly in their sleep (see my previous post on this).

Just a few years ago, I taught high school and college students (read some of my stories about it here) and the scene was completely different. They struggled to learn English. Anyone who has tried to learn a foreign language knows what I mean.

Although this is well-known, it's a bit of mystery why. It's not the case that my Chinese students didn't have the right mouth shape for English (I've heard people -- not scientists -- seriously propose this explanation before). It's also not just that you can learn only one language. There are plenty of bilinguals out there. Jesse Snedeker (my PhD adviser as of Monday) and her students recently completed a study of cross-linguistic late-adoptees -- that is, children who were adopted between the ages of 2 and 7 into a family that spoke a different language from that of the child's original home or orphanage. In this case, all the children were from China. They followed the same pattern of linguistic development -- both in terms of vocabulary and grammar -- as native English speakers and in fact learned English faster than is typical (they steady caught up with same-age English-speaking peers).

So why do we lose that ability? One model, posited by Michael Ullman at Georgetown University (full disclosure: I was once Dr. Ullman's research assistant), has to do with the underlying neural architecture of language. Dr. Ullman argues that basic language processes are divided into vocabulary and grammar (no big shock there) and that vocabulary and grammar are handled by different parts of the brain. Simplifying somewhat, vocabulary is tied to temporal lobe structures involved in declarative memory (memory for facts), while grammar is tied to procedural memory (memory for how to do things like ride a bicycle) structures including the prefrontal cortex, the basal ganglia and other areas.

As you get older, as we all know, it becomes harder to learn new skills (you can't teach an old dog new tricks). That is, procedural memory slowly loses the ability to learn new things. Declarative memory stays with us well into old age, declining much more slowly (unless you get Alzheimer's or other types of dementia). Based on Dr. Ullman's model, then, you retain the ability to learn new words but have more difficulty learning new grammar. And grammar does appear to be the typical stumbling block in learning new languages.

Of course, I haven't really answered my question. I just shifted it from mind to brain. The question is now: why do the procedural memory structures lose their plasticity? There are people studying the biological mechanisms of this loss, but that still doesn't answer the question we'd really like to ask, which is "why are our brains constructed this way?" After all, wouldn't it be ideal to be able to learn languages indefinitely?

I once put this question to Helen Neville, a professor at the University of Oregon and expert in the neuroscience of language. I'm working off of a 4-year-old memory (and memory isn't always reliable), but her answer was something like this:

Plasticity means that you can easily learn new things. The price is that you forget easily as well. For facts and words, this is a worthwhile trade-off. You need to be able to learn new facts for as long as you live. For skills, it's maybe not a worthwhile trade-off. Most of the things you need to be able to do you learn to do when you are relatively young. You don't want to forget how to ride a bicycle, how to walk, or how to put a verb into the past tense.

That's the best answer I've heard. But I'd still like to be able to learn languages without having to study them.