The best explanation which scientists may give to the result of a younger one having higher grades which is in negation to popular theory is:
This refers to the intelligence quotient which is used to measure the analytical skills and ability to solve problems, identify patterns, etc and people with high IQ are considered to be geniuses.
With this in mind, we can see that there is a popular theory that first born children tend to have higher IQs but because a younger one has better grades, scientists are likely to say that the research is probabilistic.
Read more about intelligence quotient here:
https://brainly.com/question/6352562