I have come across a useful round-up the idea of mutation accumulation as the most likely significant mechanism for rapid decline in human intelligence over the past eight or so generations:
This author came across the idea in WD Hamilton's second volume of Narrow Roads of Gene Land - just as I did:
and he fills in the background and more recent history of this idea.
My only major quibble with this article is that the first and most-important and earliest cause of mutation accumulation was the decline in childhood (i.e. pre-sexually-mature) mortality rates starting from the agricultural and industrial revolutions.
This means that mutation accumulation very probably began in Britain from 1750-1800 and was becoming measurably apparent by 1850. Apart from public health and hygiene (e.g. water supply) improvements, medical breakthroughs would only have begun to have an effect on natural selection from the early 20th century.
So mutation accumulation as a cause of dysgenesis is not an abstract and theoretical speculation about the future - something possible unless we do x, y, and z...
No! Whatever may happen from now onwards - mutation accumulation is something that has already happened, beginning several generations ago - and the product of dysgenic change is us, i.e. the modern population in developed nations: we are it.