The idealized scientist might start by questioning everything and assuming nothing. However, one usually has to make starting assumptions to get things going. For instance, David Hume proved that the notion that science works at all is founded on the un-provable assumption that the future will conform to the past (i.e., if e=mc2 yesterday, it will do so again tomorrow).
It follows from the hunch just described that hypotheses about language should put as small a burden as possible on the child’s linguistic experience and as great a burden as possible on the biologically given system, which we call Universal Grammar (UG). Of course, the role of experience is not zero, or else every detail of language would be fixed along genetic lines. Nonetheless, given that linguistics tries to explain, the null hypothesis should place the role of experience as close to zero as possible.
In contrast, there has been a strong trend in psychology — and folk science, for that matter — to assume everything is learned and prove otherwise.
Ultimately, if science proceeds as it should, we’ll all converge on the same theory somewhere in the middle. In the meantime, wildly divergent starting assumptions often unfortunately lead to folks simply talking in different languages.
A good example is a recent exchange in Trends in Cognitive Sciences. Waxman and Gelman had recently wrote an article about how children’s assumptions about the world (they called these assumptions “theories”) guide learning even in infancy. Sloutsky wrote a letter to complain that Waxman and Gelman had failed to explain how those assumptions were learned. Gelman and Waxman responded, in essence: “Who says they’re learned?”
All three are intelligent, thoughtful researchers, and so at the risk of simplifying the issue, Sloutsky’s problem with the “innate theories” theory is that nobody has given a good characterization of how those theories are instantiated in the brain, much less how evolution could have endowed us with those innate theories. Sloutsky assumes learning unless proven otherwise.
However, Waxman and Gelman’s problem with Sloutsky is that nobody has a good explanation — even in theory — of how you could learn anything without starting with some basic assumptions. At the very least, you need Hume’s assumption (the future will conform to the past) to even get learning off the ground.
Both perspectives have their strengths, but both are also fatally flawed (which is not a criticism — there aren’t any perfect theories in cognitive science yet, and likely not in any science). Which flaws bother you the most depends on your starting assumptions.