Intelligence as we think of it today, though, is very complex, encompassing hard-to-measure traits like creativity, ability to reason, communication skills and more. A person's thinking ability can also be influenced in the moment by nutrition, amount of sleep, distractions, stress and other factors.
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Nevertheless, the researchers posit that RT can indicate the inherent intelligence of a person, likely referring to that individual's genetically inherited brainpower. This is therefore not affected by things like education level, environmental influences and individual health.
At any rate, the study found that RT rates have dramatically increased over time, basically meaning people are becoming mentally more slow and stupid. Men went from 183 ms in the Victorian Era to 253 ms in modern times. Women went from 188 ms to 261ms.
I have to wonder that there was a glitch in the way that the times were measured, but will assume the increases are accurate.