I was thinking about the main environmental factors that could possibly influence such a difference in average test scores, so I compared the similarities and differences in both eastern and western types of childhood, and it came down to 1 most seemingly substantial environmental factor.
The main environmental factor is the learning of the complex East Asian written languages.
For instance, the Chinese written characters are modified symbols of many patterns found in nature as well as in human behaviour. Since IQ tests subject people to recognize complex patterns, those who have already done much practice in recognizing and remembering complex patterns found in the Chinese written language would already have developed much skill in pattern recognition, and would therefore be faster and more effective in dealing with IQ test questions that require similar recognition and manipulation of complex patterns compared to those who lack years of such practice.
I myself have learnt how to read and write the Chinese characters from a young schooling age in Singapore, so I am well aware of the substantial mental requirements it takes to memorize the complex structure of new Chinese characters in detail and then visualize their structure well enough to write them out accurately without error. The written Chinese that is taught in Singapore is the simplified version, for Chinese people growing up in East Asia such as Taiwan and China, they would be learning traditional Chinese characters that are more complex in structure.
So I think the main environmental reason for East Asians having higher average IQ scores compared to westerners and Black people is due to having a written language that is more visually complex.
The Japanese use a written language that includes many kanji characters taken from the Chinese language.
The Koreans have a written language that is similar in form.
The Ashkenazi Jews who are ranked at the same top level as East Asians in average IQ test scores also have a written language with complex symbols. So they would also have gotten much practice in pattern recognition like East Asians do.
I think the neural plasticity that the brain has to go through would be more intense and dynamic for someone learning a complex written language compared to someone learning a simpler written language such as English.
Race and IQ - The IQ data is out - in Danish television
Update: I did some research after writing this blog post and found a research study that shares my theory.
Asian kids' IQ lift: reading system may boost Chinese scores
"Learning to read 2,500 pictorial symbols, as Chinese students do in grade school, yields a 5-point advantage on IQ tests, compared with the scores of Westerners whose languages are based on alphabets, according to a new analysis of mental capabilities of Greek and Chinese children. The international team of analysts, led by psychologist Andreas Demetriou of the University of Cyprus in Nicosia, attributes the scoring disparity to a superiority in visual and spatial tasks that comes with learning to read Chinese.
"Our findings support the assumption that reading and writing systems are powerful methods for influencing the development of mental abilities, and perhaps brain growth, in individuals and in cultures," Demetriou says."