Introduction:
The relationship between intelligence and achievement has long been debated among researchers. While intelligence, as typically measured by IQ tests, has been shown to correlate positively with various measures of academic and career success, the exact nature of this relationship is complex with many external factors also playing a role. Through causal comparative research comparing groups with different intelligence levels, researchers have sought to better understand how intelligence impacts achievement when holding other variables constant. This paper will discuss several examples of causal comparative research studies that have examined the link between intelligence and achievement.
Sample Study #1 – Intelligence and High School GPA
One well-known study analyzed data from over 6,000 American high school students to compare intelligence and high school GPA (Duckworth et al., 2007). Students were divided into four equal groups based on their scores on a baseline intelligence test administered in 9th grade. Researchers then obtained the students’ actual high school GPAs upon graduation to measure achievement. As predicted, they found higher average GPAs correlated with higher baseline intelligence levels. The differences between the group means were relatively small, with an effect size of only 0.25. Additionally, there was substantial overlap between the groups, with many high-IQ students achieving average or below-average GPAs and vice versa. This suggests intelligence alone does not determine achievement and other non-cognitive factors play an important role.
Sample Study #2 – Intelligence and College Completion
A second causal comparative study examined the relationship between intelligence and college completion by following over 1,000 students from their freshman year of college through graduation (Bowen et al., 2005). Participants were grouped based on their SAT scores, a standardized test highly correlated with IQ. Researchers then tracked how many students from each group went on to complete a 4-year college degree within 6 years. As expected, they found higher SAT groups had significantly higher college completion rates – over 70% for the top group versus under 50% for the lowest. Intelligence alone did not guarantee success, as over a quarter of high-SAT students still failed to graduate. Non-academic strengths like grit, self-control, and motivation likely differentiated achievers from non-achievers within intelligence levels.
Sample Study #3 – Intelligence and Job Performance
Another study explored the link between cognitive ability and job performance for over 300 employees across diverse roles and industries (Schmidt & Hunter, 1998). Employees completed IQ-style tests and their job performance was evaluated by supervisors after 1 and 2 years on the job using standardized rating scales. As past meta-analyses had also shown, researchers found general mental ability positively correlated with most job performance criteria, with correlations averaging around 0.5. The relationships varied significantly depending on the knowledge, skills, and abilities required for specific jobs. For some complex roles heavily dependent on cognitive problem-solving, the correlations approached 0.7, but for simpler jobs they were closer to 0.3. These findings indicate intelligence impacts achievement through work, but job requirements moderate this effect.
Sample Study #4 – Intelligence in Childhood and Life Success
One longitudinal study deserves particular attention given its size, duration, and use of objective life outcomes measures beyond education and work (Strenze, 2007). Researchers assessed the IQs of over 45,000 Scottish children born between 1921-1926 at ages 5, 8, 11 and 15 using standardized mental aptitude tests. Researchers then obtained life records for these same individuals documenting metrics like occupational status, income, home ownership, physical health in older age, and even mortality records. Statistical analysis revealed higher childhood intelligence moderately but significantly predicted greater life success across a wide range of concrete, objective outcome measures when tracked over 60-80 years later. Specifically, every 15-point increase in childhood IQ corresponded to 2.4 more years of schooling completed, 25% higher income, and reduced mortality risk. Many high-IQ children did not achieve elite life statuses, and some lower-IQ children did surprisingly well – again demonstrating intelligence alone does not determine destiny.
Discussion and Conclusion:
The causal comparative research consistently shows a positive relationship between intelligence and various measures of achievement across both education and career domains. Higher intelligence generally correlates with higher GPA, college completion rates, job performance ratings, occupational status, income levels and other life success metrics. The effect sizes are moderate rather than perfect, and the relationships are far from deterministic. Many other non-cognitive factors strongly influence achievement within any given intelligence bracket. Differences between cognitive abilities alone do not fully account for variance in real-world success. Overall, while not diminishing the importance of maximizing human intellectual capital through education, these nuanced findings emphasize a balanced, multi-factorial view of achievement that considers strengths beyond narrow IQ metrics. Future research should continue exploring how multiple traits interact to produce diverse achievement outcomes across diverse demographic groups and life paths.
This paper reviewed four notable examples of causal comparative research studies that compared achievement levels across groups with different measured intelligence scores. Consistently, higher intelligence related to higher achievement levels on average. The relationships were complex with multiple other factors also influencing outcomes. Overall, intelligence provides advantages but does not singlehandedly determine destiny. A balanced, holistic view of both cognitive and non-cognitive human attributes remains most aligned with research findings on the multidimensional relationship between intelligence and achievement.
Andrew Stroud