Verdict: Unlikely.
This video is full of misinformation, cherry-picking, and other forms of bias - please don't watch it. Even just focusing on the topical question, it's not entirely clear what the claim exactly is, but I'll just go with: "Seed oils are a significant contributor to a population-wide decline in human intelligence."
Intelligence:
Human intelligence, as approximated by IQ scores, has been gradually increasing for basically the entire history of IQ tests, a phenomenon known as the Flynn Effect. While the effect itself is well-established and mainstream, its causes remain controversial. I won't go through this literature as it's too much to cover, but a few points worth making are:
- The cause is likely multi-factorial, with multiple dimensions that may change by location and time period.
- The effect is generally accepted to be environmental, rather than genetic in origin.
- A commonly hypothesized factor is improved nutrition over time (Rindermann, Becker, & Coyle, 2017).
Starting roughly in the 1990s, the effect partially reversed in some countries, known as the Reversed Flynn, anti-Flynn, or Negative Flynn Effect. It took some time for the trend to be distinguishable from statistical noise, so the first papers documenting the new trend appeared around the early 2000s.
Thus, charitably, we've had around 20 years to document and scramble for explanations of this emerging trend. However, given that we've had over 100 years to explain the rise in intelligence, and have yet to come to broad consensus, I'm skeptical that the fall will be solved faster, or be a simple 1-factor explanation.
It is also worth noting that if the Flynn Effect has an environmental cause, without changes to underlying genetics, then it should be expected that it would eventually diminish. Thus, many authors don't think that there is anything to explain (Pietschnig & Voracek, 2015; Dutton, van der Linden, & Lynn, 2016; Wongupparaj et al, 2023; Shakeel & Peterson, 2022; Pietschnig, Voracek, & Gittler, 2018; Flynn & Shayer, 2018; Trahan et al, 2014; also see Acosta, Smith, & Kreinovich, 2019).
Seed Oils:
As the video itself acknowledges, the view that vegetable oils and linonleic acid are unhealthy is not mainstream. Additionally, seed oils in particular have been disproportionately subject to conspiracy theories, so I think that some skepticism is warranted here. The video also overlooks how preparation (such as cooking and hydrogenation) affects the health impact of seed oils. Most notably, none of the major reviews or meta-analyses of the Reversed Flynn Effect (cited earlier) mention vegetable or seed oils.
Ironically, the video itself provides some evidence that seed oils are an unlikely cause of declining intelligence. This graph, likely sourced from Knobbe & Stojanoska (2017) - a poorly cited staple of seed oil myths - was featured at least 3 times in the video:
It shows that vegetable oil consumption has increased steadily during the same period of time that the Flynn Effect was observed - that is, it is more reasonable to conclude that seed oil caused an increase in intelligence, rather than a decrease. Note: I don't know how reliable this consumption data is, but other sources (eg, availability and absolute) seem to correlate.
Nutrition:
The study by Bratsberg & Rogeberg (2018) mentioned in the video, is well-received, and concludes that the causes of the Reversed Flynn Effect are also mostly environmental, rather than genetic. Virtually all of the same factors used to explain the increase in intelligence have been proposed to account for its decrease, including nutrition.
The Reversed Flynn Effect hits different countries at different times. The first large-scale study documenting a possible decline in intelligence in the USA by Dworak, Revelle, & Condon (2023) suggests the trend may have started there some time before 2006. However, the decline is not uniform across all facets of intelligence. As such, the authors note:
As the present study found differential slopes in mean cognitive
ability scores, it seems unlikely that quality of nutrition or health
would account for conflicting differences among the three-dimension
rotation tasks and the remaining tasks. Rather, we would expect to see
a reverse Flynn effect across all domains if the differences were due
to changes in nutrition or worsening health as cognitive processes
impacted by these variables are likely overlapping.
Studies in other countries have similarly suggested that declines are not uniform across all domains of intelligence or across different age groups, and therefore other factors are more likely causal of Reversed Flynn Effects (Woodley & Meisenberg, 2013; Platt et al, 2019; Lazaridis, Vetter, & Pietschnig, 2022; Pietschnig et al, 2023; Colom et al, 2023; Dutton et al, 2018; Vainikainen & Hautamäki, 2022).
Conclusion:
- We can rule out seed oils accounting for declining intelligence as a mainstream hypothesis, well-replicated phenomenon, or broad consensus - it simply does not appear in the academic literature.
- Poor nutrition in general is an unlikely candidate explanation for the Reversed Flynn Effect in the USA because it does not fit available evidence.
