Fluid and Crystallized intelligence:
- Fluid Intelligence AKA "IQ"
  - Is how easily/quickly one learns/comprehends patterns / complex tasks.
  - This is typically measured via Raven's Progressive Matrices
    
    Scores are relative, meaning they only mean something when compared to others. Scores are transformed into a normal distribution / bell-curve, where the average is a score of 100, with a standard deviation of 15 (meaning 66% of people will score between 85 and 115).
  - Babies have an IQ, else they could never learn anything.
- Crystallized Intelligence
  - Is essentially knowledge / memorization (information that one has stored in memory, regardless of how long it took one to aquire it).
  - A newborn child has zero crystallized intelligence (except for ancestral blood knowledge such as how newborn sea turtles know to rush towards the water, AKA instinct).
Heritability of IQ: 57% to 73%.
- "Genome-wide association studies establish that human intelligence is highly heritable and polygenic" - Study "Davies, G., Tenesa, A., Payton, A. et al. Genome-wide association studies establish that human intelligence is highly heritable and polygenic. Mol Psychiatry 16, 996–1005 (2011). https://doi.org/10.1038/mp.2011.85"
- "Intelligence is one of the most heritable behavioural traits ... from about 20% in infancy to perhaps 80% in later adulthood." - Study Plomin, R, and I J Deary. “Genetics and intelligence differences: five special findings.” Molecular psychiatry vol. 20,1 (2015): 98-108. doi:10.1038/mp.2014.105
Positive correlation between birth order and IQ (i.e. first-borns have higher IQs than last-borns)
How health impacts IQ
Koko the Gorilla: An abnormally intelligent gorilla. "The Gorilla Foundation" has a great write-up of how they teach sign-language and other means of communication to gorillas. Though they've never asked questions to humans.
Abnormally intelligent parrots that have asked questions to humans include Alex and Apollo.
Latter-day Saints (AKA Mormons) are selecting for slightly higher IQ partners (slightly eugenic), political conservatives for slightly lower IQ partners (slightly dysgenic), and everyone else for moreso-lower-IQ partners (dysgenic). Similarly found that conservatives, in general, show a weaker selection against intelligence than do liberals.
- Study Kirkegaard, E., Dutton, E. Will Intelligent Latter-day Saints and Smart Conservatives Inherit the Earth? Differential Selection for Intelligence in the USA Based on Religiosity and Conservatism. Evolutionary Psychological Science (2022). https://doi.org/10.1007/s40806-022-00327-y
National IQ is the best predictor of economic growth
- Study Francis, G., & Kirkegaard, E. O. W. (2022). National Intelligence and Economic Growth: A Bayesian Update. Mankind Quarterly, 63(1), 9–78. https://doi.org/10.46469/mq.2022.63.1.2
The Flynn effect
- During the 19XXs, many parts of the world experienced both fluid & crystallized intelligence score rises. The cause(s) are uncertain but suspected to possibly include:
  - additional schooling
  - improved nutrition
  - more stimulating enviroments
  - reduction in disease
  - reduction in lead-based paint
  - reduction in leaded fuels
  This effect has stopped in many developed countries, and a "Reverse Flynn effect" has been ongoing (see below).
- Reverse Flynn Effect: Since the 199Xs, IQ scores are decreasing in Norway, Denmark, Australia, Britain, the Netherlands, Sweden, Finland, and German-speaking countries. The cause is uncertain, but may include:
  - (some or all of the) Causes of the Flynn effect have been exhausted (e.g. all lead-based paint has been removed)
  - Immigration from lower-IQ higher-TFR countries
  - Dysgenic forces/trends are increasing
  - Culture (somewhat of a catch all)
  - Decline in religion
Qiran Zhao, Xiaobing Wang, Scott Rozelle "Better cognition, better school performance? Evidence from primary schools in China" China Economic Review, Volume 55, 2019
- Full paper: https://fsi-live.s3.us-west-1.amazonaws.com/s3fs-public/better_cognition_better_school_performance.pdf
- 10000 primary school students from private migrant schools in (Chinese provinces) Beijing and Suzhou, and from public rural schools in Henan and Anhui, were IQ-tested using the Raven Standard Progressive Matrices test. High rates of developmental delay (about 33% of the students have Ravens scores that are less than one standard deviation lower than an international mean). Mean Flynn-adjusted IQ Score of "88.95". Student cognition is by far the most important factor in their academic achievement.

Image credits

Image Raven's Progressive Matrices by Wikimedia user Life of Riley under license CC BY-SA 3.0
Photo Koko the Gorilla by Ron Cohn of the Gorilla Foundation under fair use.
Photo Apollo the Parrot by Victoria (Tori) Lacey and Dalton Mason under license CC BY-SA 4.0