Im sure someone more qualified can give a much better and more lengthy explanation, but knowing a fair amount about Malthusian theory and economics, the best-selling book 'The Population Bomb' written in 1968 by Paul R. Ehlrich focuses heavily on how the rise of an immense and, according to him, unsustainable middle class would cause overpopulation and eventually mass-starvation in the classic Malthusian catastrophe. He covers this quite extensively and cites how populations with large amounts of wealth and financial security, bolstered by cheap goods and the new welfare state, tend to have far, far more children because they can seemingly afford it with no repercussions. Whether or not he was right I imagine is irrelevant to your question (he clearly wasnt, we dont have mass-starvation and times have changed), but I think his work does fall under a popular image in the 60s-early 70s of what this huge middle class could mean for society.