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Some of this post has been redacted.

Context: I have worked with mentally retarded patients in a social care context and physics PhDs in a laboratory context.

██████ ███████████ ██ ███████ ██ ██ ██ ███ ████████ ██████████ █████ █ ██████████'█ ████ ($-2\sigma$). ██████ ████ ████ ███ ████████ █████████ █████ █ ██████████'█ ███████ ($-1\sigma$) ███ ██ ██████████ ███████████ ██████████. ██ █████ █████, 2.3% ██ █ ██████████ ██ ████████ ████████ ███ 84.1% ██ ███████████ ██████████.

███ ███ ██ █████ ███████ ██ ██ █████ ██ ████████ ██ ██████ ███ ████████ ████ ██ █████. ██ ███ ████ ██ ██ $+2\sigma$ ($\sigma\equiv\text{“standard deviation”}$) █████ ███ ████ ████ ██████ ████ ██ ██ ██ ███ ██ ████ (50.0% ██ ███ ██████████) ██ ████████ ████████ ████████ ██ ███. ██ ███ ███ $+3\sigma$ █████ ███ ████ ████ ██████ ████ ████ $+1\sigma$ █████ ███ ████ (84.1% of the population) ██ ████████ ████████ ████████ ██ ███ [1]. ██ ███ █████████, 13.6% ███ ████ ███ █████████ ██████████ "██████████ ██████ ███████████". ████ ████ ███████████, ███ ███ 2.3% ██ ███ ██████████ ██ ███████████ ████████.

At the time of writing, Wikipedia lists [2] the following signs of mental retardation. I have applied a strikethrough like this to arbitrary social standards (including childhood development delay) because social standards are defined by consensus. The same goes for specific example behaviors.

• Delays in reaching, or failure to achieve milestones in motor skills development (sitting, crawling, walking)
• Slowness learning to talk, or continued difficulties with speech and language skills after starting to talk
• Difficulty with self-help and self-care skills (e.g., getting dressed, washing, and feeding themselves)
• Poor planning or problem-solving abilities
• Behavioral and social problems
• Failure to grow intellectually, or continued infant childlike behavior
• Problems keeping up in school
• Failure to adapt or adjust to new situations
• Difficulty understanding and following social rules

We can bucket the remaining traits into three broad categories.

• Difficulties with speech and language skills. ███ ███████ ██████ █████ ███ █████, ██████ ████ ████████ ████████████ ███ ███ ███████ ██████████████ ███████ █████████ ████ ███████████.
• Poor problem solving abilities. Difficulty adapting to new situations. ███ ███████ ██████ ██████ ██████ ████ █████ ██████. █████████████ ███████ ██████ ██ ████ ███ ██ ███ ███████ ██████████. ████████ ██████ ██ ███ ██████ ███████ █████.
• Poor planning. Childlike behavior. Difficulty with self-care. ███ ███████ ██████ ████ ██ █████ ████ ████████ ██ ███ █████████ ██ ████ ████ █████████. ████ █████ █████████ ██ ██████████ ████████. ███ ███████ ██████ ███ ███████ ██████████████ ███ ███ █████████ █████████ ████ ████████ ███████.

All three buckets contribute to intellectual stagnation. The intellectual gap between high versus low IQ people widens as they age.

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[1] For reference, 130 ($+2\sigma$) is an average undergraduate mathematics major and 103 ($+0.2\sigma$) is an average undergraduate major in social work^[source].

[2] My list of symptoms comes from this article on Wikipedia, retrieved July 24, 2020.