Flynn Effect Cohort Comparisons

I'm going to follow up a little further to my previous two posts and talk about making cohort comparisons under a Flynn effect.

It's quite common for researchers to talk about the Flynn effect advantage (or disadvantage) of one population cohort compared to another. For instance, in Dickinson & Hiscock (2010) and Agbayani (2011), this type of statement is frequent. Unfortunately for these researchers, a Flynn effect cohort comparison is not technically correct. In the best of circumstances it can be ambiguous, and as it is typically done, it's wrong.

The best way to see this is to understand first that the Flynn effect works across time, not across cohorts. Nearly every empirical study involving the Flynn effect shows that raw intelligence scores tend to increase universally and uniformly over time, no matter the age group, no matter the population. Whoever you are, wherever you are, no matter how old you are, what the Flynn effect cares about is how much time has passed. If you make a comparison across ten years of time, then you'll get ten years worth of Flynn effect. If you make a comparison across fifty years of time, you'll get fifty years worth of Flynn effect. If you make a comparison across zero years of time, then you'll get zero years worth of Flynn effect (which is to say, no Flynn effect at all).

So what does this mean for a cohort comparison? Let me return to the idealized chart of data I used in Intelligence as Field, and let's examine the raw intelligence history of two cohorts within it, the population born at time 100 and the population born at time 140.

Age	Raw Intelligence Scores by Age and Year
	95	40.8	41.6	42.5	43.3	44.2	45.1	46.0	46.9	47.9	48.8	49.8
	85	45.6	46.5	47.5	48.4	49.4	50.4	51.4	52.4	53.5	54.6	55.7
	75	50.4	51.4	52.5	53.5	54.6	55.7	56.8	58.0	59.1	60.3	61.5
	65	54.0	55.1	56.2	57.3	58.5	59.7	60.9	62.1	63.4	64.6	65.9
	55	55.8	56.9	58.1	59.2	60.4	61.7	62.9	64.2	65.5	66.8	68.1
	45	57.6	58.8	59.9	61.2	62.4	63.7	64.9	66.2	67.6	68.9	70.3
	35	58.8	60.0	61.2	62.4	63.7	65.0	66.3	67.6	69.0	70.4	71.8
	25	60.0	61.2	62.4	63.7	65.0	66.3	67.6	69.0	70.4	71.8	73.3
	15	51.0	52.0	53.1	54.2	55.2	56.4	57.5	58.7	59.8	61.0	62.3
	5	15.0	15.3	15.6	15.9	16.2	16.6	16.9	17.3	17.6	18.0	18.3
		105	115	125	135	145	155	165	175	185	195	205
	Year

The assumption that gets blindly made by researchers is that the birth year 140 cohort (BY140) is more intelligent than the birth year 100 cohort (BY100), due to the Flynn effect. But that's technically incorrect, or at the very least, it's technically ambiguous. The result of the comparison is going to very much depend on how the comparison gets made.

If the comparison is across time, then yes, BY140 will always emerge as more intelligent than BY100. For instance, if we are comparing the 25 year-olds of BY140 to the 25 year-olds of BY100, then there is forty years of time span between those comparison points, and thus forty years worth of Flynn effect. The same result happens if we compare the 65 year-olds of BY140 to the 65 year-olds of BY100. Cohort comparisons that span across time will always reveal the specific impact of a Flynn effect.

But that's not how researchers typically do it.

Researchers typically make their comparison at a particular point in time. For instance, researchers will say something like, while comparing BY100 to BY140 at time 165, a Flynn effect adjustment must be made to account for the Flynn effect disadvantage BY100 has relative to BY140. Or it might be put this way: the relative scores of BY100 and BY140 at time 165 are distorted due to the Flynn effect.

Such statements are pure nonsense.

In any comparison made at a particular point in time, the Flynn effect disappears. To repeat: the Flynn effect works across time, not across cohorts. If the cohort comparison being made involves no span of time, then there can be no Flynn effect distinction. Period.

As I mentioned in Intelligence as Field, there seems to be an odd sense floating about within the research community that one's Flynn effect is established once and for all at one's birth. It is this odd sense that must be leading people to assume that the Flynn effect works across cohorts and not across time. But the empirical data does not support that assumption. The empirical data shows that the Flynn effect continues to work equally at all places and at all times—it works throughout a person's entire lifetime, it is not established once and for all at one's birth. Therefore, the Flynn effect is not a function of birth or of cohorts, it is purely a function of time.

The mistakes I'm pointing out here are extremely common within the intelligence research community, and truth be told it's a shame. These mistakes mask the true nature of the Flynn effect, a phenomenon which has very much to tell us about the nature of human intelligence. But the Flynn effect is only going to speak to us if we don't first go out of our way to misunderstand it.

References

Agbayani, K. A. (2011). Patterns of age-related IQ changes from the WAIS to WAIS-III after adjusting for the Flynn effect. Retrieved online from http://repositories.tdl.org/uh-ir/handle/10657/236.

Dickinson, M. D. & Hiscock, M. (2010). Age-related IQ decline is reduced markedly after adjustment for the Flynn effect. Journal of Clinical and Experimental Neuropsychology, 32(8), 865-870.