A poll of experts taken in 1983 showed that about half concurred that doubler should have at least four hearts and at least four spades; a similar poll taken in 2000 showed 80% in concurrence.
The passage describes the evolution of expert opinion on the minimum heart and spade count for a "doubler" over time.  A survey in 1983 showed half agreed, while a similar poll in 2000 found 80% agreed.