Truncation Selection by 50% (half the population)

This post uses the results of: "Boosting IQ by 15 points (truncation selection)". We now look at another example.

Suppose we had a country whose population originally shared the Caucasian average IQ,

(mean = 100; std dev = σ = 15).

And suppose a catastrophe occurred which led to half the population emigrating - such things have been known to happen in European history.

And suppose the brightest were the ones who emigrated.

What would be the average IQ of those who remained?

• The proportion who remain, p, is 50%.
• From the table at the bottom of the post above, the intensity of selection, i(p) = 0.8
• Then use this equation, S = σ * i(p).

The average IQ of those left behind is S = 15 * 0.8 = 12 points below the mean; i.e. the non-emigrating have an average IQ of 88. However, due to regression to the mean, subsequent generations will do better than this.

Their descendants will have an IQ of R = h²S, where h² = 0.6 (say) is the additive heritability of IQ.

So R = 0.6 *12 = 7 IQ points below the original population mean.

Their descendants will have an average IQ of 93.

Here's a list of country IQs.

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The power of population genetics ...

Diagram from here

The mean IQ of Ireland was documented in the country list above as 92.

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You have to be careful with country IQs. If the country is not ethnically homogeneous you tend to get a stratified society where the average IQ hides more than it illuminates. For example, in Israel the Ashkenazim are smart and tend to dominate at the top of society - but non-Ashkenazim have a more typical Middle-Eastern IQ and numerically dominate - the resulting averaged IQ is documented as 95. Many Latin-American countries are ethnically stratified so one number is not that useful.

If the country has had a dysfunctional economic system and/or history (China is a case in point, Vietnam another), then deprivation will depress IQ scores.