I don't think they are intelligent. An intelligent person will never live a life of a scammer. He will find a way to bring bread and butter in a legitimate way so that the business can sustain for longer term and make a positive impact for him and his family.
Most scammers are indeed smooth-brained and low IQ, but a few cases stand out as deviations from the norm. For example, Bernie Madoff comes to mind. In his case, though, everything started out as legitimate, but after he stopped being able to produce genuine returns for his clients, he decided to turn full scammer, just taking the funds from new clients to pay the old. Madoff is indeed simple an exception to the rule.
The whole thing reminds me of a quote by theymos:
scammers are often pretty stupid