It's really surprising stuff - but does not of course support the conclusions of the green ink brigade.
The Piagetian (i.e. based on Piaget's observations of childhood developement) test "volume & heaviness" explores 11-year-old pupils' appreciations of these two concepts. For instance, pupils are shown a 5x4x3 block of plasticine, which is lowered into a beaker full of water until it is just under water: the pupils see the water overflow. The plasticine is removed, the beaker is refilled, and pupils are asked whether more, less or the same amount of water would overflow if the plasticine was lowered down to (a) the middle and (b) the bottom of the beaker. There are 13 other tests exploring other aspects of volume and heaviness.
What Shayler found was that scores were steady from 1976 to 1995, and then declined rapidly. Boys, who had been better at these tests, are now the same level as girls.
Shayler speculates that this is due to the lack of experiential play in school, and the rise of the video game culture.
But does it matter? Life has changed. Doubtless today's kids are better a some things than kids of 30 years ago, due to their gaming and tech skills. Claiming that this points to a destroyed childhood or a dumbing down of Britain is wildly overstating the case. Perhaps, though, it points to one reason why take-up in science education is falling - kids don't get the basic concepts the way they used to. There's other evidence that points that way too - and it should be easy enough to remedy in school if that's really what's happening.