The Flynn
effect refers to generational IQ test norm changes first systematically described in the 1980s (Flynn, 1987). IQ
changes have been observed to be positive (on average 3 IQ points per decade),
but differentiated according to the investigated country and the intelligence test
domain. Paradoxically, larger gains for fluid than for crystallized intelligence have been observed. In a recent meta-analysis by Pietsching and Voracek (2015), the
progress and strength of the Flynn effect since the introduction of
psychometric intelligence testing in the early 20th century till 2013 was analyzed along with the moderating
influences of age, economic growth, sample health status, and sex.
The key
findings of the study can be summarized as follows:
- Strong evidence for continuous global generational IQ test score gains in the general population over the past century (2 SD or 0.28 IQ points annually) were observed.
- Gains in fluid IQ were substantially stronger than those in crystallized IQ (4.1 vs. 2.1 IQ points per decade).
- The IQ change trajectories showed robust evidence for decreasing gains in recent decades.
- Stronger gains were observed for adults than for children, showing large effects for fluid and spatial IQ.
- Gross domestic product growth was positively associated with full-scale, crystallized, and spatial IQ but it showed negligible effects for fluid IQ.
- IQ gains were not observed for psychometric g.
The
authors also provided an overview of possible explanations and theories for the
observed gains, dividing them into environmental, biological, and hybrid (i.e.,
interacting biological and environmental). The environmental factors include
education, technology, decreasing family size (dysgenic fertility), and test-taking behavior. It
is assumed that the availability of education and technology for individuals
from different socioeconomic backgrounds had a beneficial influence on the
level of intelligence. This also had some influence on becoming more familiar
with tests, mainly multiple choice, and by that
changing test-taking behavior –
principally increasing guessing.
Among
biological factors, hybrid vigor is often mentioned, which refers to the mating of individuals from genetically
dissimilar subpopulations, thereby increasing allelic heterozygosity and
reducing homozygosity.
Hybrid
factors include decreased blood lead levels, genomic imprinting (epigenetic
inheritance), nutrition, and reduced pathogen stress. Moreover, more complex factors such as reduced IQ variability, effects of social
multipliers, and decreasing life history speed (fewer offsprings), have been
proposed in the literature.
The end of the
Flynn effect?
In more recent
decades a stagnation or even a decrease of the Flynn effect, mainly in more
developed Western countries, has been reported. It is possible that the beneficial effects have caused the IQ increase to reach a
ceiling. Some explanations have also linked these decreases to a higher
proportion of immigrants from less developed countries. The table below summarizes
the findings.
Study
|
Country
|
Findings
|
Sundet et al. (2004)
|
Norway
|
Substantial gains in intelligence
were observed from the mid-1950s (test years) to the end 1960s–early 1970s,
followed by a decreasing gain rate and a complete stop from the mid-1990s.
The gains seemed to be mainly caused by decreasing prevalence of low scorers.
|
Teasdale
& Owen (2005; 2008)
|
Denmark
|
Intelligence
test results from over 500,000 young Danish men, tested between 1959 and
2004, showed that performance peaked in the late 1990s, and has since
declined moderately to pre-1991 levels. A contributing factor in this recent
fall could be a simultaneous decline in proportions of students entering
3-year advanced-level school programs for 16–18 year olds.
|
Woodley
& Meisenberg (2013)
|
Netherlands
|
Sixty-three observations of
secular IQ changes were collected from three demographically diverse studies
of the Dutch population for the period 1975–2005 (representing the 1950–1990
birth cohorts). Declines due to the anti-Flynn effect were estimated at -4.52
points per decade, whereas gains due to the Flynn effect were estimated at
2.18 points per decade. The N-weighted net of these is a loss of -1.35 points per decade, suggesting an
overall tendency towards decreasing IQ in the Netherlands with respect to
these cohorts.
|
Dutton
& Lynn (2013)
|
Finland
|
The
average IQs of approximately 25,000 18–20 year old male military conscripts
in Finland per year were analyzed for the years 1988 to 2009. The results
showed increases in the scores on tests of shapes, number and words over the
years 1988 to 1997 averaging 4.0 IQ points a decade. From 1997 to 2009 there
were declines in all three tests averaging 2.0 IQ points a decade.
|
Dutton
& Lynn (2013)
|
France
|
The results of the French WAIS
III (1999) and the French WAIS IV (2008–9) were compared based on a sample of
79 subjects aged between 30 years and 63 years who took both tests in
2008–2009. It is shown that between 1999 and 2008–9 the French full scale IQ
declined by 3.8 points.
|
Pietsching
& Gitter (2015)
|
Germany
and Austria
|
A
meta-analysis (k=96; N=13,172) showed an inverse u-shaped trajectory of IQ
test performance changes (initial increases and subsequent decrease of performance) over 38
years (1977–2014).
|
The paradox of
the Flynn effect
The
cognitive functions that predict psychometric intelligence are thought to be working memory and processing speed. The idea behind the relation
between processing speed and g is rather straightforward. Processing speed is
viewed as a form of cognitive or neural limitation of processing a simple
stimulus. The most often studied
parameters are reaction time and reaction time variability. Similarly, working memory and
intelligence are highly related constructs. In a recent confirmatory analysis
it was shown that working memory shared 83.4% of variance with fluid
intelligence (Chuderski, 2015). Hence, with respect to the Flynn effect, one
would expect that reaction time would over the years decrease, while working memory capacity would show an
increase. However, the findings do not follow these trajectories.
Silverman
(2010) analyzed simple visual reaction time (RT) obtained in a study conducted
by Francis Galton in the late 1800s with the RTs obtained in subsequent
studies. In Galton’s study, the median RTs obtained by men and women between
ages 18 and 30 were 183.0 and 187.0 ms, whereas the RTs obtained in more recent
studies (3,836 men and 3,093 women) were for men 250.43 ms (SD = 46.53) and 277.71 ms (SD = 30.76) for women. Moreover,
the RTs obtained in the comparison studies were all longer than the RTs
obtained by Galton. Out of several possible causes for longer RTs, Silverman suggested
that the most tenable were that RT has been increased by the buildup of
neurotoxins in the environment and by the increasing numbers of people in less
than robust health who have survived into adulthood.
Based on
Silverman’s findings, Woodley et al. (2013) in their meta-analytic study of RT
concluded that the Victorians were cleverer than modern populations. Woodley
and colleagues used the data on the
slowing of simple reaction time described in a meta-analysis of 14 age-matched
studies from Western countries conducted between 1889 and 2004. Using
psychometric meta-analysis, they computed the true correlation between simple
reaction time and g, yielding a decline of −1.16 IQ points per decade or −13.35
IQ points since Victorian times. Further, the difference between the meta-regression
trend-weighted present (2004) simple RT mean (270.18 ms) and the trend weighted
1889 mean (193.17 ms) was 77 ms. Woodley and colleagues further concluded that the most likely reason
for this finding, also put forward by Silverman, was that those with poorer
health and slower RTs surviving into adulthood are more numerous in the
modern era than in the past.
Miller
(1956) suggested that the typical short-term memory capacity (STMC) of an adult
is 7 plus or minus two objects. Cowan (2005) suggested that the typical
working memory capacity (WMC) of an adult is 4, plus or minus one object.
Were these numbers lower in the past? This would be expected based on the Flynn
effect. To answer this question, Gignac (2015) analyzed digit span (forward and backward) across 85 years of data (Ns of 7,077 and 6,841). The mean adult verbal STMC was
estimated at 6.56 (±2.39), and the mean adult verbal WMC was estimated at 4.88
(±2.58). No increasing trend in the STMC or WMC test scores was observed from
1923 to 2008. This finding may be considered surprising, since memory span is
so intimately related to fluid intelligence. At the moment they still lack a
plausible explanation.
We can
conclude that (Gignac, 2015, p. 93) “it
may be prudent to acknowledge that the magnitude, pervasiveness, and true
nature of the Flynn effect remains a substantially open question”.
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