In a very small nutshell, his explanation for the Flynn Effect is this. Human potential at birth is unchanged; we are not, in any fundamental sense, becoming a smarter species. But the way we live has changed. IQ tests were first established in the 19th century at a time when daily life was concrete and practical. The tests, however, had to be abstract to make them culturally neutral. People, therefore, found them harder because they were unaccustomed to such modes of thought.
In the 20th century, greater educational possibilities combined with technological advances introduced abstract thought into daily life. It takes, for example, a high degree of abstract thinking to operate a mobile phone or computer. People became better at IQ tests and, steadily, the scores rose. So IQ scores are meaningless unless their date and social norms are taken into account. This leads to Flynn’s grandest and most fervently held view — that a lack of social awareness leads inexorably to folly. Indeed, the penultimate chapter is a list of 14 examples in which science has failed because of social blindness. Low-IQ people, for example, are not more prone to violence and, contrary to widespread assumptions, no clear link between nutrition and IQ has been found.