It is but how long does it take? I do not believe it is as simple as saying:

Person A has an IQ of 102 Person B has an IQ of 104, therefore; the child will have an IQ of 103. There is no specific IQ gene and so I would imagine that it would take just as long from an evolutionary standpoint to decrease the average IQ as it would for us to loose our tails and stop living in tree's. It certainly wouldn't happen in a matter of 30 years. At least, I don't think it would... I could be wrong but I don't believe I am.