I have good memories of 1975. I got my first secure job, a Lectureship in Psychology at the Middlesex Hospital Medical School, part of the University of London. It was a glorious summer, followed the next year by an even better and drier one, and I finally finished my PhD. Little did I realise that we had reached peak intelligence, and after that it would be downhill all the way. In my defence, it takes time to notice that a peak has been passed, and all this relates to Norwegian data, but nonetheless, the endullment of Western society was underway, and any subjective concerns I had about the increasing foolishness of the world have now been amply confirmed. 1975 turns out to have been a pivotal year, and a new paper says that the reasons are within the family.
Flynn effect and its reversal are both environmentally caused
Bernt Bratsberg and Ole Rogeberg
Population intelligence quotients increased throughout the 20th century—a phenomenon known as the Flynn effect—although recent years have seen a slowdown or reversal of this trend in several countries. To distinguish between the large set of proposed explanations, we categorize hypothesized causal factors by whether they accommodate the existence of within-family Flynn effects. Using administrative register data and cognitive ability scores from military conscription data covering three decades of Norwegian birth cohorts (1962–1991), we show that the observed Flynn effect, its turning point, and subsequent decline can all be fully recovered from within-family variation. The analysis controls for all factors shared by siblings and finds no evidence for prominent causal hypotheses of the decline implicating genes and environmental factors that vary between, but not within, families.
This is an interesting and quite complicated paper, which argues that because the Flynn effect is of equal magnitude in older and younger sons within the same family, then it was probably caused by unknown factors affecting all family members, and cannot have been caused by differences between families. For example, if poor families have more children, and these tend to be dull, then there will be differences between families, not within them.
The method seems to be simple: every year test the intelligence of the first born son and compare it with the intelligence of the second son within that same family, the second son of course being born and being tested some time later. If both sons show the same pattern of rising or falling intelligence, then the Flynn effect, whatever its cause, is due to things which affect all families in the same way. Clean water, good food, home computers, healthy living can boost all family member’s ability levels. Genetic differences would favour some families over others, but that is not detected in this study, so is probably unlikely.
To narrow down the set of hypotheses, we examine the extent to which we can recover observed Flynn effects from within-family variation in large-scale administrative register data covering 30 birth cohorts of Norwegian males. Within-family variation will only recover the full Flynn effect if the underlying causal factors operate within families. Notably, if within-family variation fully recovers both the timing and magnitudes of the increase and decline of cohort ability scores in the data, this effectively disproves hypotheses requiring shifts in the composition of families having children. This set of disproved hypotheses would include dysgenic fertility and compositional change from immigration, the two main explanations proposed for recent negative Flynn effects
A metareview of empirical studies argues that the positive Flynn effect relates to improved education and nutrition, combined with reduced pathogen stress. Turning to the negative Flynn effect, the metareview notes a deceleration of IQ gains in some studies and suggests that these may relate to (i) decreasing returns to environmental inputs (saturation) or (ii) the picking up of effects that cause IQ decreases and may ultimately reverse the Flynn effect, such as dysgenic fertility. Dysgenic fertility is also the favored hypothesis in a recent literature review on reversed Flynn effects, where the authors conclude that dysgenic trends are the simplest explanation for the negative Flynn effect. A negative intelligence–fertility gradient is hypothesized to have been disguised by a positive environmental Flynn effect, revealing itself in data only once the ceiling of the Flynn effect was reached.
So, this paper confirms what was already known, that the secular rise in intelligence test scores has already given way to a fall in those scores (with perhaps a slight rise again?), but attempts to rule out a whole set of possible explanations, saying that there is no need for them. The full effect can be shown to operate within families.Hence the authors’s view of the significance of their findings.
Using administrative register data with information on family relationships and cognitive ability for three decades of Norwegian male birth cohorts, we show that the increase, turning point, and decline of the Flynn effect can be recovered from within-family variation in intelligence scores. This establishes that the large changes in average cohort intelligence reflect environmental factors and not changing composition of parents, which in turn rules out several prominent hypotheses for retrograde Flynn effects.
Now for the detail. These are conscription data for young men. No reason to believe young women would be different. The authors report that the family average tracks the pattern of the first born pretty well. However, as the years went by, lower scoring first borns were unlikely to have had younger brothers who were tested themselves, and this requires a correction factor to be applied. Although the correction for this selection bias is carefully done, it introduces some uncertainty to the estimates, though the authors see this as well within reasonable limits. The authors also argue, in my view convincingly, that the reversal of the Flynn effect is so rapid that it is unlikely to be a dysgenic indicator, since such things happen between successive generation, not within one.
A salvo has been fired against genetic factors being implicated in the apparent drop in intelligence. As a note of caution, it would be good to look at whether this holds true of all components of the intelligence test: numbers, words and shapes. Anyway, the authors say that these data do not allow them to specify what causes the Flynn effect and its reversal, merely to suggest it cannot be genetics.
The strong points of the study are that it is based on conscript age data obtained on almost all Norwegian men from 1962 to 1991, and they restrict themselves to Norwegians with two Norwegian parents, so they will not be cultural or genetic confounders.