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It is well-established that, through most of the twentieth century, Ashkenazi Jews (that is, Jews from central Europe, around Germany) had an IQ of about 115 - that is, one standard deviation above the British average IQ of 100 which is used as a reference.
This is been plausibly linked to the proportionately much greater number of Jews who attained intellectual eminence through the 20th century - especially in mathematics and the sciences.
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This differential Jewish pre-eminence emerged throughout the 19th century, and this emergence has almost-always been put down to their emergence from the ghetto at about that time, and the gradual and partial elimination of barriers against Jews in Europe occupying high status social positions.
But now that evidence has emerged for a rapid and substantial and of decline of intelligence from sometime in the 19th century, it is possible that the differential of intellectual achievement between Ashkenazi Jews and the European populations that surrounded them may be partially or mostly explained by an opening-up of an intelligence gap: due to the decline of the gentile population having begun a few generations before the same process took hold among Jews.
In other words, it is possible that the 19th century Jewish 'emergence from the ghetto' was either a coincidence, or else was itself a consequence, of the opening-up of differentially greater intelligence among the Ashkenazim.
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Thus, current evidence is that Victorian English had about the same intelligence as middle-late 20th century Ashkenazim.
So one explanation was that initially English and Ashkenazi intelligence were pretty much the same; but that the decline in English intelligence (with, presumably, much the same story of decline in France, Germany, the USA etc) led to a situation where Ashkenazim emerged through the twentieth century as having differentially higher intellectual achievement than other European-descended populations.
Nowadays, the Jewish-European intelligence gap may have narrowed, and intermarriage has become much more frequent; but the main trend is that both Jewish and European intelligence are now declining; such that the most obvious thing about modern human accomplishment is that there is so little of it; with an almost complete absence of geniuses.
Obviously, the above is an hypothesis, which invites empirical investigation; but it has some plausibility in terms of being a parsimonious explanation of the rise and decline of Ashkenazi Jewish genius in relation to other European groups.
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Note: Even if Europeans and Ashkenazim started out - say 1750 - having a very similar level of general intelligence - this does not mean that creative genius was identical in the two groups. Indeed it almost certainly was not; since personality (high Psychoticism) is the other main factor along with very high intelligence in enabling genius; and Jews and gentile Europeans were two substantially separate populations living in significantly different societies, and therefore experiencing different selection pressure with respect to personality traits.
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Further Note: But if it is true and Jewish intelligence was indeed significantly higher than gentile European intelligence in - say - 1800, then this implies that average Ashkenazi intelligence circa 1800 would have been around two standard deviations higher than modern average intelligence - in other words an average intelligence of around-about 130. This would be startling, but not, I think, impossible; since some 'intellectual families' of high social class nowadays apparently have about that level of average intelligence.
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