For those who are new to these topics, the general factor (symbolized by the letter g and always written in lower case, even when the first word in a sentence) is a hypothesized variable that is said to cause all mental abilities to positively correlate and was discovered by Charles Spearman. g is not without controversy. For example, this blog has been greatly enhanced by the critical comments of “Duke of Leinster”, a self-proclaimed descendant of the Unsinkable Molly Brown and self-proclaimed participant in Steve Hsu’s prestigious BGI study, with a self-reported old SAT score four standard deviations from the mean (one in 30,000 level) and self-reported perfect score on ALL THREE SECTIONS of the old GRE (I’ve never even heard of that happening).

I seldom believe people when they tell me they have extremely high IQ’s so this could all be bogus, however the fact that the entire behavioral genetics elite blogosphere is so terrified of this man that he’s been banned from virtually every one of those blogs except this one, and the fact that with the exception of a commentator named “JK”, no one has dared to even try to refute his technical arguments against heritability (even though tons of behavioral genetics people are reading) suggests the Duke’s IQ is way up there and he might have inherited high IQ genes from Molly Brown who used her quick wit to survive the Titanic. So let’s read what Duke has to say about g:

g is NOT a THING. In fact, g, as originally defined by Spearman, does NOT exist. All that exists is a higher order factor or a principal component of a PARTICULAR battery for a PARTICULAR population. Get it?

This shows good conceptual understanding. g is simply whatever variable(s) are causing all mental tests to positively correlate in a particular population at a particular time. So in theory g could have a completely different meaning in different samples. In theory g could just be how much cultural exposure you’ve had to IQ tests or how much effort you put into them.

And yet, scholar Arthur Jensen felt that IQ differences were especially meaningful if they were correlated with g and such correlations are now known as Jensen effects (a term coined by scholar J. Philippe Rushton). For example Jensen felt that inbreeding depression and the brain size IQ correlation were Jensen effects, but the Flynn effect and IQ gains caused by adoption were not. Jensen did however feel there was a biological component to the Flynn effect related to increased 20th century brain size which he did consider a Jensen effect. The implication was that Jensen effects reflect real biological differences in intelligence and non-Jensen effects simply reflect culturally acquired knowledge, skills, or cognitive habits.

However groundbreaking recent research by scholars James R. Flynn, Jan te Nigenhuis and Daniel Metzen found this not to be the case showing that real and important biological IQ differences caused by fetal alcohol syndrome, prenatal cocaine exposure, and traumatic brain injury are not Jensen effects. As usual, scholar Richard Lynn (probably the most theoretically correct IQ expert in history), was years ahead of his time on this issue too, writing way back in 2006 that although sex differences in IQ are apparently not Jensen effects, they are real biological differences in intelligence nonetheless.

So while it’s possible for a non-Jensen effect to be biological, I’m aware of no case of a Jensen effect being non-biological or even non-genetic. A good example of a genetic Jensen effect is chronometrics (measures of physiological reaction speed scored in milliseconds) which have actually shown a substantial slowing over the 20th century, despite all the modern environmental advances that have been propping up IQ scores (and perhaps in the case of nutrition) propping up aspects of intelligence itself.