Imagine that instead of intelligence, what we really cared about in our society was fishing aptitude. At age 10, a child is given tests that involve fishing while wading in a stream, fishing while standing on a bridge, fishing while riding in a boat on a river, and fishing while in a boat on the ocean. Let us suppose that a child's percentile in any one test is highly correlated with that child's percentile on all of the other tests. For example, a child who is in the 85th percentile for catching fish when wading in a stream is likely to be in the 85th percentile for catching fish in the other situations. We could say that this child has a relatively high Fishing Quotient, or FQ.
Common folk wisdom might be that one intuitively identifiable subset of the human race, called Good Ol' Boys, has a higher FQ on average than another subset, called City Fellers. Suppose that results of FQ tests bear out this folk wisdom. In that case, some researchers might try very hard to identify genetic differences between Good Ol' Boys and City Fellers, and they might try very hard to tell "Just-So" stories about how those genetic differences might explain the difference in average FQ. But we should be cautious about concluding that the intuitively identifiable differences reflect fundamental biological differences or that those biological differences are what explain FQ.
Racial differences in average FQ would be one indication that FQ is innate. More powerful evidence would be:
--the fact that someone's FQ as a child is highly correlated with that individual's FQ as an adult, indicating that the environment has relatively little effect on FQ.
--the fact that policy interventions intended to change FQ, including many experiments with fishing education, fail to make a significant difference in FQ.
--the fact that identical twins raised apart tend to have about the same correlation of FQ as identical twins raised together.
One argument against the innateness of FQ would be the Flynn effect, which says that re-norming has occurred over time. In this context, the Flynn Effect is that on two of the tests--say, the boat on the river and the boat on the ocean--the number of fish that you need to catch to be in the 50th percentile has gone up gradually but steadily over the past hundred years.
Some researchers say that this means that the environment affects FQ. Other researchers say that this means that performance on those two tests are not comparable over time, but that FQ is still innate. There are bound to be many indicators of FQ that change over time. For example, fishing income is correlated with FQ, but fishing income can change over time for many environmental reasons. The fact that any one indicator changes over time does not prove that FQ is not innate. In the words of Linda S. Gottfreson "No one would mistake a thermometer for heat."
A theoretical way to rescue environmental determinants of FQ is the Dickens-Flynn hypothesis. This hypothesis is that children with a bit of aptitude for fishing are given lots of opportunity and encouragement for fishing, so that small innate advantages lead to large differences in FQ.
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