Are IQ Levels in Decline?

Check out this answer from Consensus:

The evidence suggests that IQ levels may indeed be in decline, reversing the historical trend of the Flynn Effect. Various factors, including preterm birth, medical conditions, cognitive reserve, and educational attainment, influence this decline. Understanding these factors and their impact on cognitive development is crucial for developing interventions to mitigate IQ decline and promote cognitive health across the lifespan.

The question of whether IQ levels are in decline has garnered significant attention in recent years. This article aims to explore the evidence surrounding this issue by examining various research studies that have investigated changes in IQ over time, factors influencing these changes, and the implications of these findings.

Historical Trends in IQ

Historically, the phenomenon known as the “Flynn Effect” described a substantial rise in IQ scores over the 20th century, typically about 3-5 IQ points per decade in developed countries. This increase has been attributed to various biological, social, and educational factors2. However, recent evidence suggests that this trend may have reversed. For instance, a study involving over 500,000 young Danish men showed that IQ performance peaked in the late 1990s and has since declined to pre-1991 levels2. This reversal of the Flynn Effect indicates a potential decline in IQ levels in recent decades.

Factors Contributing to IQ Decline

Several studies have identified specific factors that may contribute to the observed decline in IQ levels. For example, children born preterm have shown a significant decline in IQ scores from childhood to adolescence, even in the absence of clear neurological damage3. This decline has been associated with specific neural regions, such as the parietal and temporal lobes, and highlights the vulnerability of preterm children to cognitive decline.

Similarly, youth with type 1 diabetes have demonstrated a greater decline in verbal IQ (VIQ) and full-scale IQ (FSIQ) compared to healthy controls over a 12-year period. Factors such as younger age at diabetes onset and a history of hypoglycemic seizures were associated with this decline5. These findings suggest that certain medical conditions and their management can negatively impact cognitive development.

Cognitive Reserve and Terminal Decline

The concept of cognitive reserve posits that individuals with higher IQs can better tolerate age-related brain pathologies, leading to a delayed onset of terminal decline (TD). However, once TD begins, the rate of decline is steeper for those with higher IQs1. This phenomenon was observed in a study where higher IQ was associated with a delay in the onset of TD by approximately 1.87 years for perceptual-and-motor-speed and 1.96 years for verbal ability1. These findings provide partial support for the cognitive reserve hypothesis and demonstrate that IQ can significantly moderate cognitive change trajectories in old age.

Educational Attainment and Cognitive Decline

Educational attainment has also been shown to influence cognitive decline. A longitudinal study of the Glostrup 1914 cohort revealed that individuals with higher education levels exhibited higher initial cognitive performance but also experienced steeper declines in IQ over a 35-year period8. Despite the steeper decline, higher mean IQ was observed among participants with formal education across all ages, suggesting that education provides a cognitive advantage that persists throughout life.

Specific Populations and IQ Decline

Certain populations are particularly vulnerable to IQ decline. For instance, children with early unilateral brain injury have shown significant declines in IQ over time, with larger lesions associated with lower initial IQ scores and smaller lesions linked to greater declines6. Additionally, individuals with childhood-onset schizophrenia have demonstrated postpsychotic declines in full-scale IQ during adolescence, primarily due to an inability to acquire new information and skills rather than a dementing process7.

 

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Are IQ levels in decline?

Guy Madison has answered Near Certain

An expert from Umeå university in Psychology

Intelligence is the ability to learn and solve problems. IQ tests is the most common way to quantify intelligence, but does not lend itself well for comparing across groups or across time. This is because the IQ metric is relative, defined as a percentile of a certain reference population, according to its original purpose to identify individuals in need of support (Binet & Simon, 1904). Specifically, IQ tests are standardised every few decades or so in relation to a representative sample of a whole country’s population, which is given a mean of 100 and a standard deviation of 15.

Nevertheless, changes could be observed within the same standardisation period or by comparing standardisations across time. More recent standardisations require higher test scores for the same IQ, suggesting an increase in performance corresponding to 18 IQ points in Western countries from about 1950 to 2000 (Flynn, 2012). This trend has ended or reversed in most Western countries, but continues in less developed countries (Lynn & Becker, 2019). The questions are therefore how these trends reflect actual ability, environmental factors, and genetic factors.

IQ tests are good at measuring intelligence when everything else is constant, but not when conditions change systematically. For example, having taken a test increases performance on subsequent tests, which suggests that the experience with tests in general and abstract reasoning in general inflates test results. Increased years of schooling, regular cognitive testing, and the widespread use of information technology are factors that might increase test performance without affecting the underlying genotypic ability (Dickens & Flynn, 2001; Woodley of Menie & Fernandes, 2015).

Better indices of intelligence are needed for comparing across time, preferably absolute rather than relative, objective rather than subjective, and closely related to real-life performance. Reaction time and other chronometric performance is negatively associated with intelligence on what seems to be a fundamental neural level (Ullén, Forsman, Blom, Karabanov, & Madison, 2008; Ullén, Söderlund, Kääriä, & Madison, 2012). Increasing reaction times indicates decreasing intelligence across several different samples and time periods (Woodley, te Nijenhuis, & Murphy, 2014; Ullén et al., 2012; Woodley, Madison, & Charlton, 2014; Madison, Woodley of Menie, & Sänger, 2016). Real-life performance has also declined, in terms of innovation rate (Woodley & Figueredo, 2013), vocabulary (Woodley of Menie, Fernandes, Figueredo, & Meisenberg, 2015), and educational attainment (Chen, Chen, Liao, & Chen, 2013; Wang, Fuerst, & Ren, 2016). This is consistent with a negative association between individuals’ intelligence and the number of children they have (Woodley of Menie, Figueredo, Dunkel, & Madison, 2015; Vining, 1995; von Stumm, Batty, & Deary, 2011), which leads to a decrease in intelligence across generations. The co-occurrence model (Woodley & Figueredo, 2013) accounts for this combination of environmentally driven increase in test performance and genetically driven decrease in underlying ability.

In conclusion, detailed analyses indicate a decrease in genotypic intelligence corresponding to 0.5 – 1.5 IQ points per decade, whether based on reaction time (Woodley et al., 2014; Woodley et al., 2014) or differential fertility (Woodley of Menie & Fernandes, 2015; Woodley of Menie et al., 2015; Woodley of Menie & Fernandes, 2016).

References

Binet, A. & Simon, T. (1904). Méthodes nouvelles pour le diagnostic du niveau intellectuel des anormaux. L’année Psychologique, 11, 191-244.

Chen, H.-Y., Chen, Y.-H., Liao, Y.-K., & Chen, H.-P. (2013). Relationship of fertility with intelligence and education in Taiwan: A brief report. Journal of Biosocial Science, 45, 567-571.

Dickens, W. & Flynn, J. R. (2001). Heritability estimates versus large environmental effects: The IQ paradox resolved. Psychological Review, 108, 346-369.

Flynn, J. R. (2012). Are we getting smarter?: Rising IQ in the twenty-first century. Cambridge: Cambridge University Press.

Lynn, R. & Becker, D. (2019). The intelligence of nations. London: Ulster Institute for Social Research.

Madison, G., Woodley of Menie, M. A., & Sänger, J. (2016). Secular slowing auditory simple reaction time in Sweden (1959-1985). Frontiers in Human Neuroscience, 10, 407.

Ullén, F., Forsman, L., Blom, Ö., Karabanov, A., & Madison, G. (2008). Intelligence and variability in a simple timing task share neural substrates in the prefrontal white matter. The Journal of Neuroscience, 28, 4239-4243.

Ullén, F., Söderlund, T., Kääriä, L., & Madison, G. (2012). Bottom-up mechanisms are involved in the relation between accuracy in timing tasks and intelligence – further evidence using manipulations of state motivation. Intelligence, 40, 100-106.

Vining, D. R. (1995). On the possibility of the reemergence of a dysgenic trend with respect to intelligence in American fertility differentials: an update. Personality and Individual Differences, 19, 259-263.

von Stumm, S., Batty, G. D., & Deary, I. J. (2011). Marital status and reproduction: Associations with childhood intelligence and adult social class in the Aberdeen children. Intelligence, 39, 161-167.

Wang, M., Fuerst, J., & Ren, J. (2016). Evidence of dysgenic fertility in China. Intelligence, 57, 15-24.

Woodley of Menie, M. A. & Fernandes, H. B. F. (2015). Do opposing secular trends on backwards and forwards digit span evidence the co-occurrence model? A comment on Gignac (2015). Intelligence, 50, 125-130.

Woodley of Menie, M. A. & Fernandes, H. B. F. (2016). The secular decline in general intelligence from decreasing developmental stability: Theoretical and empirical considerations. Personality and Individual Differences, 92, 194-199.

Woodley of Menie, M. A., Fernandes, H. B. F., Figueredo, A. J., & Meisenberg, G. (2015). By their words ye shall know them: Evidence of negative selection for general intelligence in vocabulary usage since the mid 19th century. Frontiers in Psychology, 6, 361.

Woodley of Menie, M. A., Figueredo, A. J., Dunkel, C. S., & Madison, G. (2015). Estimating the strength of genetic selection against g in a sample of 3520 Americans, sourced from MIDUS II. Personality and Individual Differences, 86, 266-270.

Woodley, M. A. & Figueredo, A. J. (2013). Historical variability in heritable general intelligence: Its evolutionary origins and socio-cultural consequences. Buckingham, UK: The University of Buckingham Press.

Woodley, M. A., Madison, G., & Charlton, B. G. (2014). Possible dysgenic trends in simple visual reaction time performance in the Scottish Twenty-07 cohort: a reanalysis of Deary & Der (2005). Mankind Quarterly, 55, 110-124.

Woodley, M. A., te Nijenhuis, J., & Murphy, R. (2014). Is there a dysgenic secular trend towards slowing simple reaction time? Responding to a quartet of critical commentaries. Intelligence, 46, 131-147.

 

Are IQ levels in decline?

Hynek Cígler has answered Uncertain

An expert from Masaryk University in Psychology, Quantitative Psychology, Psychometrics, Intelligence

The great overview of so called Flynn’s effect (and also for “reverse Flynn’s effect) describing the changes in average intelligence tests score is well described in the Pietschnig and Voracek’s (2015) metaanalyses, which reviewed all the available studies about changes in IQ scores.

Almost the whole 20th century, the average achievement in intelligence test was increasing. However, in last few decades, decrease in western countries was observed – but in many other countries, IQ scores are still probably increasing. If we are therefore speaking about “humans getting dumber”, it should be first specified, which humans and where. It also depends on which “kind of intelligence”; the Flynn effect influence different intelligence domains in a different way.

As the main cause of Flynn effect, the mentioned metaanalysis pointed out so called “life history speed”, which is related to family size (smaller families have slightly more clever children), education, quality of nutrition, stress etc. It is obvious that these characteristics have not been changing in western countries in last decades, and moreover, some of them are getting “worse”, which could lead to lover intelligence.

And finally: the decrease is (still) so small, that it shouldn’t have any observable influence on practical life. The newspapers headlines looks much more alarming than it should look like.

 

Are IQ levels in decline?

Nachshon Meiran has answered Uncertain

An expert from Ben-Gurion University of the Negev in Intelligence

My only addition to previous answers on the Flynn effect is that a recent paper (now online first in the journal Intelligence) shows that it works in opposite directions on high and low intelligence groups. Intelligence went up in the high group and went down in the low group (showing a trend of increased individual differences)

 

Are IQ levels in decline?

Leehu Zysberg has answered Uncertain

An expert from Gordon College of Education in Education, Psychology

I have recently written an article examining evidence for this possibility from the PISA international test system and the evidence is worrying but equivocal. While some countries (most of the developed countries with well established educations systems) do show regression in their academic achievements and IQ scores, others do not, while some show a moderate increase. What is going on then?

My educated guess would be something called “regression to the mean” – the underlying assumption is that statistically high (and surely very high) performance cannot be maintained throughout a long period of time, as measures slowly drop closer to the general mean and fluctuate to and fro above and around the mean. It makes sense – you cannot run your fastest ALL the time. A slow runner will from time to time have a surprise sprint where he outperforms himself and the opposite will happen to a very fast one – he WILL HAVE “bad days”. I am quite confident this is what we are witnessing right now, but more time and additional evidence are required to make better sense of it all.