In an epic telephonic conversation I had with Michael Woodley yesterday, he came up with a brilliant, clear and useful metaphor to explain why it is that although it might take (say) 24 generations to raise general intelligence by one standard deviation, it has only taken about 6 generations to reduce intelligence by the same amount.
'Eugenic' selection to increase intelligence is like walking at 5mph on a 4mph treadmill going in the opposite direction.
This is because eugenic selection is a mutation-selection balance mechanism, in which natural selection is working against existing levels of inteligence due to a significant level of spontaneous deleterious genetic mutations; such that most of selection is simply a process of filtering-out the continually recurring mutations.
Most of natural selection is thus a Red Queen phenomenon of running fast just to stay in the same place.
So, intelligence-reducing mutations are the 4mph adverse treadmill which natural selection is walking against - but positive selection for higher intelligence was so powerful during the medieval era (with near zero reproductive success for those of low intelligence) that intelligence increased at (say) 5mph - so that there was progress at a rate of 1mph.
But suppose that natural selection stopped working to increase intelligence, and indeed began to work against intelligence - what would happen?
Well, even if natural selection merely stopped, and did not reverse, supposing that nothing more happened than natural selection ceased to filter-out the spontaneously occurring deleterious mutations - then this would correspond to the person walking more and more slowly, and then standing-still on the treadmill: then the treadmill would sweep the person backwards at 4mph.
This merely by the stopping of natural selection to favour intelligence, there would be a rapid decline in intelligence.
But in addition, to the 4mph backwards, which would by itself rapidly undo hundreds of years of intelligence gains - the past 6-8 generations in England (and The West generally) have also seen a reversal of natural selection for intelligence - a dysgenic pattern of selection - such that the most intelligent have the lowest reproductive success, and vice versa.
SO - the reason that intelligence is declining so rapidly is that we are walking on a treadmill of spontaneous deleterious mutations, and the treadmill is tending to push intelligence backwards at (say) 4mph - but instead of walking forwards we have now turned around and started walking backwards - in the same direction as the treadmill.
We have added dysgenic selection to the already existing underlying tendency for intelligence to decline due to spontaneous mutations.
We have turned around on the treadmill and started walking briskly in the wrong direction - adding our (say) 3 mph walking pace to the underlying 4mph of the treadmill.
And that is one way to understand why it is much quicker to undo intelligence gains than it was to generate them in the first place.
Note: This is obviously just a metaphor to get a specific point across! Don't, please, get hung-up on the fact that - presumably - this thought-experiment treadmill must be miles long... This is a ladder to be used to ascend to a higher level of understanding, then kicked-away.
An alternative metaphor could be that natural selection for intelligence is swimming upstream against the flow of deleterious mutations.
For some hundreds of years in medieval times, the speed of swimming was upstream faster than the flow of the river downstream and the swimmer advanced relative to the river bank. But over the past couple of hundred years Western man has turned-around and begun to swim downstream - adding the speed of his swimming to the speed of the current - and the river bank is now whizzing past.
It will not take long for the downstream swimmer to get back to the point at which he began his gradual journey upstream, and probably he will overshoot it; to find himself a lot further downstream than where he began.