There's a saying that "Fish is brain food." While it's true that a Mediterranean diet that's low in fat and red meat and includes a lot of lean fish has been associated with better cognition and memory in older adults, it's a bit more complicated than just eating more fish to get smarter.
A recent survey of children in China suggested something quite similar, however. Data on children between the ages of 9 and 11 who were part of the China Jintan Cohort Study was the subject of a study by researchers from the University of Pennsylvania, Brigham and Women's Hospital and the National Institutes of Health.
In their analysis of the data, there was a correlation between consumption of fish and standard IQ tests. Children who "always" ate fish scored an average of 4.80 points higher on a Chinese version of the Wechsler Intelligence Scale for Children-Revised than children who "rarely" had fish. For those children who only ate fish "sometimes" there was still a correlation to higher scores. Children who had fish occasionally showed scores that were about 3.31 points higher than their non-pescatarian playmates.
While the study included over 500 children, which is a significant sample size, there are some caveats about the work involving more detail than "eating fish makes you smart." The study was observational. That means that the frequency of fish consumption was reported by the parents and caregivers of the children. Observational studies have their place in research, but they cannot definitively establish causation. Another issue is that there was no consistency in what was meant by "fish" in the food questionnaires. Fish like mackerel, salmon, and trout are high in omega-3s. These long chain fatty acids are known to be beneficial to cell membranes in the brain. In adults, diets that include omega-3s have been correlated with larger volumes of gray matter. The study did not ask respondents to be specific about what kinds of fish were included in the diet of their children.
The research also showed a correlation between better sleep patterns and fish consumption in this age group; however, the same limits applied to that result. Parents and caregivers reported how long their children slept, on average, and how often and for how long they woke up at night. While it's easy for parents of infants to know when they are asleep and when they are awake, with older children it's not always possible to know exact sleep patterns and times.
Finally, there was no data on the families of fish eaters or nonfish eaters. Factors like socioeconomic status, activities, sports, amount of physical exercise, family size, birth order and dozens of other considerations were not a part of the research. The difference in IQ and sleep quality could have been due to any combination of these pieces of information. While most fish is healthy and barring any dietary concerns or allergies, is an excellent addition to meals, it's not quite entirely accurate to say that children who eat fish will always score higher on assessments and sleep better.