Exercise & Fluid Intelligence

Approximately 7 Minutes Reading Time

Brief Article Overview

• General intelligence (g) can be subdivided into two categories:

• 1. General crystallized intelligence (gc).
  2. General fluid intelligence (gf).
• General crystallized intelligence relies on prior learning and knowledge to solve problems, whereby general fluid intelligence is the ability to navigate novel situations.
• Fluid intelligence peaks much earlier (early adulthood), at which time it declines relatively rapidly.
• Of all potential interventions (including memory/puzzle/problem-solving training etc.), regular exercise has been shown to be unique in its ability to improve retention and reduce the rate of decline of fluid intelligence throughout adulthood.

Introduction

Intelligence is a vast subject and discussions as to the validity of current assessments, definitions and terms is not within the scope of this article. However, in order to explain the relationship between exercise and fluid intelligence, we should understand some basic definitions, to give context to the discussion.

General Intelligence (g) summarises the positive correlations amongst an individual’s various cognitive abilities. In layman’s terms, it is a measure of general performance amongst a variety of seemingly different tasks.

For example, on the surface, one’s ability to speak Latin doesn’t predict one’s mathematical prowess. However, it has been identified that the competence required to learn a complex skill, such as Latin, may translate (if somewhat partially) to one’s ability to learn mathematics.

G and IQ are both obtained by using intelligence tests (WAIS – Wechsler Adult Intelligence Scale; RPM – Raven’s Progressive Matrices or KAIT – Kaufman Adolescent and Adult Intelligence Test). These tests include various subsets, such as working memory, visual-spatial processing, quantitative reasoning, general knowledge and fluid reasoning. The g factor is essentially the correlation amongst these different subsets and IQ is the rank ordering of the overall result amongst all others who have taken the test.

A good analogy is that of a computer processor. G would represent the average speed at which an individual processor can perform a wide range of functions. Whereas IQ would be a comparison of the results of specific simulations run by the computer, and rank-ordered amongst all other computers.

It is useful to note g is highly correlated with IQ (R-value around 0.95!), to the point that they are often used interchangeably.

Fluid Intelligence and Crystallized Intelligence

In the 1960s, the theory that general intelligence can be subdivided into two categories; fluid (gf) and crystallized (gc) intelligence, was proposed by psychologist Raymond Cattell (1). After further statistical research over the next couple of decades, it seemed that his theory held some validity, and could be reliably tested for (2, 3 & 4). They are defined and distinguished as follows:

“Fluid intelligence is the ability to use logic and solve problems in new or novel situations without reference to pre-existing knowledge. Crystallized intelligence is the ability to use knowledge that was previously acquired through education and experience.”

“Fluid general ability shows more in tests requiring adaptation to new situations, where crystallized skills [domain-specific knowledge] are of no particular advantage”.

Gf and Gc Time-Trends Throughout Life

Due to the reliance on prior learning and contextual knowledge, crystallized intelligence tends to grow throughout childhood and adulthood and peaks later in life (as late as 60-70y/o). Its crystallized nature seems to be more stable over time. Fluid intelligence, however, increases throughout childhood and adolescence, but seems to peak in early adulthood and then, unfortunately, starts to decline (5, 6, 7, & 8).

Interventions to Retain Fluid Intelligence

Fluid intelligence is highly advantageous in our rapidly evolving work environments. The ability to adapt to novel situations and create effective solutions is a valuable commodity. So, how can we delay this relatively early peak? And/or reduce its decline throughout adulthood?

Brain/Memory training? There are a host of puzzles and games which promise to keep your mind “fighting fit” by exposing it to various verbal and non-verbal tests. It’s logical to assume that the more you use your brain to solve problems, the better it will become at solving problems. Unfortunately, when it comes to fluid intelligence, this is easier said than done!

As fluid intelligence is our ability to creatively and flexibly grapple with the world in ways that do not explicitly rely on prior learning or knowledge (9) and generate, transform, and manipulate different types of novel information in real-time. Most forms of cognitive training will fall into the category of prior learning or knowledge. Crystallized intelligence will improve, but fluid Intelligence will remain lacking.

It seems, therefore, that these “brain games” can improve your performance at the specific task at hand, but these skills don’t readily transfer into other aspects of cognitive ability. Researchers from the University of Oslo, who conducted a meta-analytical review on the matter concluded that; “memory training programs appear to produce short-term, specific training effects that do not generalize.” (10)

What about physical activity? It’s quite well established that physical activity can improve cognitive function (11 & 12) and reduce the risk of neurodegeneration (13) across all age groups, genders and various populations. But does this also apply to fluid intelligence?

A paper of interest was derived from the Whitehall II Prospective Cohort Study (14). In 1985, 10,385 British civil servants aged between 35-55y/o (beyond the fluid intelligence “peak”) were assessed for, amongst many other things, physical activity levels and cognitive functioning (including fluid intelligence). This study lasted 14 years, in which 5 phases of testing were performed. Of the 10,385 participants, 7,830 made it through the full study (not a bad dropout rate!). The relationship between physical activity and fluid intelligence was distilled and summarised as follows:

“In analyses adjusted for all of the covariates assessed (age, gender, education, employment grade, self-rated health, blood pressure level, cholesterol level, smoking status, mental health status, social network index score), including a proxy for baseline cognitive functioning (Mill Hill Vocabulary Scale), physical activity remained significantly associated with fluid intelligence (AH 4-I score) and phonemic fluency.”

They go on to conclude:

“These findings support the suggestion from earlier research
that physical activity moderates the decline in cognitive functioning typically associated with ageing.”

Pretty cool right? So, it seems that regular physical activity could be a significant contributing factor when trying to hang on to fluid intelligence! As if we needed another excuse to tout the benefits of exercise!

Since then a plethora of studies has supported the positive correlation between physical activity on fluid intelligence in both development and retention throughout all stages of life (15, 16, 17 & 18).

Potential Mechanisms

Brains thrive off of movement, in fact, a large proportion of their function is dedicated purely to motor output!

Interesting fact: Sea sponges lost their need for brains partly because they don’t need to physically move in order to obtain food. As brains are expensive to run, they got rid of them altogether! Sea Squirts also simplify their nervous system when they settle on the seabed and stop moving.

Physical activity modulates hormones, neurotrophins, and neurotransmitters, as well as intra- and extra-cellular pathways that regulate the expression of specific genes which encode for desired adaptations (19).

The proposed biological mechanisms in which both resistance and aerobic exercise can reduce the decline in cognitive function include, but are not exclusive to:

1. Angiogenesis; new blood vessels grow from preexisting vessels, improving blood flow to the brain, which provides the oxygen and essential nutrients, such as glucose and ketones, and removing waste products (20).
2. Mitochondrial biogenesis; increased number of cerebral mitochondria, which improve oxygen utilization, energy production and removal of waste products back into the blood (21).
3. Neurogenesis; new neural cells are created which can be used to enhance existing neural structures (22).
4. Synaptogenesis; increased formation of synapses between neurons within the brain, enhancing brain capacity and firing rate (23).

Summary

We often make the mistake of separating body and mind. But scientific research is consistently supporting the link between regular activity and cognitive function.
Since the dichotomy of general intelligence into its fluid and crystallized branches, research regarding their development, retention and decline has been of interest. Fluid intelligence, as opposed to crystallized intelligence, seems to peak earlier in life and is difficult to “train and retain”.

An unexpected correlation between physical activity and fluid intelligence then emerged. Further research supported that this correlation may be a causal relationship and therefore promoted physical activity as a promising intervention for the development of and retention of this specific category of intelligence.

This realisation adds to the myriad of benefits that regular exercise induces. We hope this appeals to those who identify as intellects and may not properly prioritise physical activity as a means of supporting their cognitive endeavours.

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Disclosure: This article is not to be used as medical advice. If you are currently experiencing physical or mental health issues, please seek professional advice from a fully qualified Nutritionist, GP or Physiotherapist.

References

1. Cattell, R. B. (1963). Theory of fluid and crystallized intelligence: A critical experiment. Journal of Educational Psychology, 54(1), 1–22.
2. Horn, J. L., & Cattell, R. B. (1966). Refinement and test of the theory of fluid and crystallized general intelligences. Journal of Educational Psychology, 57(5), 253–270.
3. Guilford, J. P. (1980). Fluid and crystallized intelligences: Two fanciful concepts. Psychological Bulletin, 88(2), 406–412.
4. Horn J.L. (1982) The Theory of Fluid and Crystallized Intelligence in Relation to Concepts of Cognitive Psychology and Aging in Adulthood. In: Craik F.I.M., Trehub S. (eds) Aging and Cognitive Processes. Advances in the Study of Communication and Affect, vol 8. Springer, Boston, MA.
5. Walter R., C., Vivian, Overton, & Willis. (1975). Fluid and Crystallized Intelligence in Young Adulthood and Old Age. J Gerontol. 30(1):53-5.
6. Kaufman A. & Horn J. (1996). Age changes on tests of fluid and crystallized ability for women and men on the Kaufman Adolescent and Adult Intelligence Test (KAIT) at ages 17–94 years. Archives of Clinical Neuropsychology. 11(2): 97-121.
7. Isingrini M. & Vazou F. (1997) Relation between fluid intelligence and frontal lobe functioning in older adults. International Journal of Aging Human Development. 45(2):99-109.
8. Hartshorne J.K & Germine L.T. (2015) When does cognitive functioning peak? The asynchronous rise and fall of different cognitive abilities across the life span. Psychological Sciences. 26(4):433-43.
9. Lesley J. Tranter & Wilma Koutstaal (2008) Age and Flexible Thinking: An Experimental Demonstration of the Beneficial Effects of Increased Cognitively Stimulating Activity on Fluid Intelligence in Healthy Older Adults. Aging, Neuropsychology, and Cognition. 15(2): 184- 207.
10. Melby-Lervåg, M., & Hulme, C (2012) Is Working Memory Training Effective? A Meta-Analytic Review. Developmental Psychology. Advance online publication.
11. Bherer, L., Erickson, K.I. & Liu-Ambrose, T. (2013) A Review of the Effects of Physical Activity and Exercise on Cognitive and Brain Functions in Older Adults. Journal of Aging Research. Volume 2013, Article ID 657508, 8 page.
12. de Greeff, J.W., Bosker, R.J., Oosterlaand, J., Visscher, C. & Hartmana, E (2018) Effects of physical activity on executive functions, attention and academic performance in preadolescent children: a meta-analysis (2018). Journal of Science and Medicine in Sport. 21(5): 501-507.
13. Hamer, M. and Chida, Y (2009) Physical activity and risk of neurodegenerative disease: a systematic review of prospective evidence. Psychological Medicine. 39(1): 3-11.

14. Singh-Manoux A, Hillsdon M, Brunner E, Marmot M (2005) Effects of physical activity on cognitive functioning in middle age: evidence from the Whitehall II prospective cohort study. Am J Public Health. 95(12): 2252-8.
15. Christensen, H, Mackinnon, A (1993) The Association between Mental, Social and Physical Activity and Cognitive Performance in Young and Old Subjects. Age and Ageing, 22(3): 175–182.
16. Reed, J.A., Einstein, G., Hahn, E., Hooker, S.P., Gross V.P. & Kravitz, J (2010) Examining the Impact of Integrating Physical Activity on Fluid Intelligence and Academic Performance in an Elementary School Setting: A Preliminary Investigation. Journal of Physical Activity and Health. 7(3)343–351.
17. Fedewa, A.L., Ahn, S., Erwin, H. & Davis, M.C (2015) A randomized controlled design investigating the effects of classroom-based physical activity on children’s fluid intelligence and achievement. School Psychology International. 36(2): 135-153.
18. Powell, R.R & Pohndorf, R.H (1971) Comparison of Adult Exercisers and Nonexercisers on Fluid Intelligence and Selected Physiological Variables. Research Quarterly. American Association for Health, Physical Education and Recreation. 42(1): 70-77.
19. Di Liegro, C.M., Schiera, G., Proia, P., Di Liegro I. (2019) Physical Activity and Brain Health. Genes (Basel). 17;10(9).
20. Morland, C., Andersson, K.A., Haugen, O.P., Hadzic, A., Kleppa, L., Gille, A., Rinholm, J.E, Palibrk, V., Diget, E.H., Kennedy, L.H., Stølen, T., Hennestad, E., Moldestad, O., Cai, Y., Puchades, M., Offermanns, S., Vervaeke, K., Bjørås, M., Wisløff, W., Storm-Mathisen, J. & Bergersen, L.H (2016) Exercise induces cerebral VEGF and angiogenesis via the lactate receptor HCAR1. Nature Communications. 8(15557):1-9.
21. Steiner J.L., Murphy, E.A., McClellan, J.L., Carmichael, M.D. & Davis, J.M. (2011) Exercise training increases mitochondrial biogenesis in the brain. Journal of Applied Physiology. 111(4):1066-71.
22. Trinchero, M.F., Buttner, K.A., Sulkes Cuevas, J.N., Temprana, S.G., Fontanet, P.A., Monzón- Salinas, M.C., Ledda, F., Paratcha, G., Schinder, A.F (2017) High Plasticity of New Granule Cells in the Aging Hippocampus. Cell Reports. Cell Reports. 21(5): 1129-1139.
23. Dietrich, M.O., Andrews, Z.B. & Horvath, T.L (2008) Exercise-Induced Synaptogenesis in the Hippocampus Is Dependent on UCP2-Regulated Mitochondrial Adaptation. Journal of Neuroscience. 28(42):10766-10771.

Bibliography

1. Intelligence: Its Structure, Growth and Action By R.B. Cattell