Research indicates that tests of "g" are the best single predictors of job performance, with an average validity coefficient of .55 across several meta-analyses of studies based on supervisor ratings and job samples. The average meta-analytic validity coefficient for performance in job "training" is .63. The validity of "g" in the highest complexity jobs (professional, scientific, and upper management jobs) has been found to be greater than in the lowest complexity jobs, but "g" has predictive validity even for the simplest jobs. Research also shows that specific aptitude tests tailored for each job provide little or no increase in predictive validity over tests of general intelligence. It is believed that "g" affects job performance mainly by facilitating the acquisition of job-related knowledge. The predictive validity of "g" is greater than that of work experience, and increased experience on the job does not decrease the validity of "g".