With longitudinal survey design of survey research method, this study
investigated the complex relationships between intelligence and academic achievement in
Mathematics and English Language over a period of four years in Nigeria. A random sample
of 637 Junior Secondary III students, aged 14 years, was drawn and followed till the end of
their Senior Secondary III at the age of 17. Their Mathematics and English Language academic
achievement were validly measured with Junior Secondary Certificate Examination (JSCE)
scores and Senior Secondary Certificate Examination (SSCE) scores. Their IQ was validly and
reliably measured with Culture Fair Intelligence Test that has been validated and standardized
for use in Nigeria. Data were collected during the students’ 2011/2012 JSCE and 2014/2015
SSCE. The IQ and JSCE as well as SSCE scores were subjected to partial correlation analysis
at 0.05 alpha, using SPSS Version 22. Results showed statistically significant relationship
between IQ and Mathematics achievement (0.499 and 0.495) when English Language is
partialled out; and between IQ and English Language achievement (0.411 and 0.346) when
Mathematics is partialled out; respectively across the junior and senior secondary levels of
schooling. Results, among others, further indicated overwhelming evidence of stability of
intelligence (0.702) with the four-year time interval in super corroboration of fluid and
crystalized theory of intelligence. Coefficient of partial determination unveiled that IQ
accounts for 24.90% to 24.50% of the variance in Mathematics achievement, and 16.89% to
11.97% of the variance in English Language achievement.