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Earlier quoted context omitted.
Oddly: this coincides with the period in which leaded petrol has largely been phased out on most countries. The use of lead in motor-vehicle fuel is though to have shaved a few points off IQ by itself. Much as with some other widespread phenomena that come to mind, it's an observed trend the reverse of what might otherwise be expected .
Lead exposure can damage the reproductive system, so perhaps it’s to be expected if a whole generation of parents were exposed to leaded gasoline. Theoretically this effect would wear off with the next generation, unless there was durable damage to genetics that gets passed on. A cursory search suggests this may be possible, but I’m not a biologist or geneticist.
Those might be behavioural, cultural, environmental substances, information/media stressors, or others. More research is needed....