While I view IQ through the same lens I view stuff like Myers Briggs tests - one possible way of describing and organizing data vs. a proscriptive, universal truth - I do have an interesting correlate to this graph that I think about sometimes.
I am a psychotherapist whose primary influences come from humanistic psychology (Maslow/Rogers), Life Span development (Erikson), with modalities informed by CBT, and more broadly, Acceptance and Commitment Therapy. While CBT and ACT have theoretic components, they're more applied in nature, while the former two are more theoretical in nature. If I'm not mistaken, CBT emerged in the 70s and 80s, and ACT is an evolution of CBT whereas Erikson and the Humanists were much earlier.
I'm mentioning all of this because I have been thinking recently about whether human intellect has peaked and did so in about the 1960s. In psychology, there really hasn't been a whole lot of new schools to emerge in the last 40-50 years, mostly just syntheses of existing schools. Most of the people I reference in discussing broad existential concepts with my patients are theorists from the early 20th century. While this is mostly a hunch, it is something I've seen represented in a number of different ways.
Obviously, this could be purely bullshit - I've not done much to confirm this notion. For all I know we've just mapped out all of the ways to understand psychological functioning and it only makes sense to focus on refining it. Perhaps I'm just ill informed of more modern theory.
I would also argue that there's another correlate to the drop in average IQs during this era - pollution. Whether it's plastics, androgen receptor blockers used as flame retardants, or even noise/electronic pollution flooding our environment with interference or distraction, the 1960s were a genesis point for plastics, modern computing, and increasing toxic environmental contaminants used in manufacturing and left to seep into our food/water.
There's been some indication that non-binary or trans-gender may be linked so some of this stuff as well. During pregnancy, hormone releases occur at specific times which signal the development of sex organs. While the blueprint is already encoded, it's not until hormones signal to the fetus to begin developing in one direction or another. It's a known issue that we use androgen interfering chemicals in a widespread manner due to their effectiveness as flame retardants. They're sprayed onto almost anything that is flammable and mass-produced. We are exposed to these things constantly. In isolated exposure cases, they are supposed to be present in levels below the "NOAEL" (no observed adverse event level). The tricky thing is that products aren't specified with the expectation that you'll have concurrent exposure from dozens of vectors in addition to the one that's being approved. While your rug might be within a safe range, your rug plus your curtains, plus your sofa, etc. cannot be calculated. These androgen receptor blockers have been shown to cause metabolic disturbances resulting in obesity, diabetes, and impaired cognitive functioning when high enough exposure is present.
Androgen receptor blockade may also play into the disruption of hormonal signaling that is being viewed as a possible cause for transgender identity. As signaling is delayed, the fetus develops along a certain pathway, only to have the signalling trigger sexual organ development later in the process, thus presenting as one sex, but with the experience of having developed for some time either without sex or as the incorrect sex. While some of the increase in transgender identities could be the result of more information available giving people the means to self-reflect in a way that was previously unheard of, this is unlikely to be the sole or primary reason.
Finally, I would argue that there is some connection between the increase in ADHD and Autism Spectrum Disorder that could be linked to some of these very same factors. There is a poorly studied and understood relationship between people with ADHD, ASD, and genderqueer identities, the overlap has been noted for some time within peer to peer analysis, but almost never studied by the wider scientific community. I am of the belief that electronic/noise pollution, environmental pollution, as well as an increased awareness of these conditions and availability of ways to manage them, are all contributing factors.
I honestly believe that these are all related to the same factors which have lead to lowered IQ averages in that time. It's hard to see impacts from within the effect window, but as time goes on we'll have a better sense of just how some of these factors maybe contributing to cognitive and hormonal disruptions.
