It has long been assumed that general mental ability was something that got you high marks at school and in tertiary courses, but ceased to have any relevance in the real life world of work. Frank Schmidt and John Hunter have shown otherwise by revisiting the data from 85 years of research involving thousands of employees in hundreds of diverse jobs.
 

What did the researchers look at?

Schmidt and Hunter compared a wide range of selection methods which had been used in making decisions about hiring, training and developing people.
 

What did the results reveal?

In studying large samples and the cumulative results of many smaller studies, the relative importance of general mental ability (gma) became clear: 

• How well a person performs a job - any job - is best predicted by gma.
  
  
• gma not only matters during training and the early stages of employment where new knowledge is being acquired, but in a worker's ongoing learning and career development as well.
  

General mental ability was also found to be the best predictor of occupational level attained by workers - as jobs increase in complexity, so do the gma scores of job incumbents.

Whatsmore, when there is a mis-match between an employee's gma and the complexity of the job they are performing, workers of lower gma move down to less complex jobs, while workers of higher gma move up to more intellectually challenging jobs.

What about experience, personality and other factors?

• It's a person's interests that best predict which occupation they'll choose to enter, but the role of interest ends there.
  
  
• Job experience is valuable for performance among newly hired workers but the correlation starts dropping off as the worker gains experience.
  
  
• When it comes to personality, while conscientiousness is a good predictor of job success, personality traits are not found to be as predictive of achievement as gma.
  
  
• General mental ability can also predict job performance better than tests of specific aptitudes that may appear on the surface to more closely target the skills required to perform a job.
   
  

Why is gma so important?

"People who are higher in gma acquire more job related knowledge and acquire it faster" say Schmidt and Hunter.

Even apparently intellectually undemanding jobs involve a surprisingly high amount of job-specific information and therefore high gma positively impacts on job performance in these jobs as well.

Knowing what one should be doing and how to do it depends strongly on gma, and higher levels of job knowledge lead to higher levels of job performance, the authors conclude.

Schmidt F. and Hunter J. General Mental Ability in the World of Work: Occupational Attainment and Job Performance.Journal of Personality and Social Psychology 2004; 1: 162-173