One of the difficult challenges when discussing the productivity of dynamic languages is to make any sort of proof about said productivity. I believe this is because it's one of those synergistic, emergent systems, where everything taken together produces a surprising or unexpected result. But this makes it hard to come up with any kind of proof, and as a result we have a bunch of people who are primarily just speaking about their personal experiences trying to convince people who haven't had such experiences that it's a Good Thing ("Hey man, you gotta try some of this stuff!"). In our business, we are constantly being shilled into trying new products, technologies, etc., most of which have been a waste of time, some of which have been a collosal waste of time AND money. So we've learned to ignore most of these claims.
Oliver Steele offers his theory about why dynamic languages are more productive, folding in issues about testing, in Test versus Type.
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