The Verbal Numerical Reasoning test (VNR) is a thirteen item test given to people in the UK Biobank. It’s kind of like the SAT, except instead of taking 2 hours, it takes 2 minutes.
Given such brevity, I’m not surprised at its low genetic loading in most studies. Short tests seldom load high on the general factor (g) of IQ tests, and g is the most genetic component of IQ tests.
So how g loaded is the VNR? One way to try to answer this is to look at its correlation with brain size (corrected for age and ethnicity)
When highly g loaded IQ tests are used, the correlation between brain size and IQ is 0.4 (maybe a bit higher (0.45) when sex is controlled and a bit lower (0.35) when sex is not controlled as in this study). And yet the VNR only correlates 0.177 with brain size, roughly half the 0.35 you’d expect from a highly g loaded (0.9+) IQ test, suggesting the VNR has a g loading of only half as high as the most g loaded tests (0.45 instead of 0.9).
With a g loading of only 0.45, it’s not surprising so few “genes” for IQ have been found.
It’s also worth noting that the VNR has a test-retest correlation of 0.65. Not bad for a 2 minute test, but nowhere near the 0.9+ stability coefficients that SATs or professional IQ tests show over a four year span.