measured in childhood predicts labor market earnings. Factors related to measuring intelligence Some authors have suggested that unless an individual is truly interested in a problem presented on an IQ test, the cognitive work required to solve the problem may not be performed owing to a lack of interest. These authors have contended that a low score on tests that are intended to measure fluid intelligence may reflect more of a lack of interest in the tasks than an inability to complete the tasks successfully. Development across life span Fluid intelligence peaks at around age 20 and then gradually declines. This decline may be related to local atrophy of the brain in the right cerebellum, a lack of practice, or the result of age-related changes in the brain. Crystallized intelligence typically increases gradually, stays relatively stable across most of adulthood, and then begins to decline after age 65. The exact peak age of cognitive skills remains elusive. Fluid intelligence and working memory Working memory capacity is closely related to fluid intelligence, and has been proposed to account for individual differences in gf. The linking of working memory and gf has been suggested that it could help resolve mysteries that have puzzled researchers concerning the two concepts. Neuroanatomy According to David Geary, gf and gc can be traced to two separate brain systems. Fluid intelligence involves the dorsolateral prefrontal cortex, the anterior cingulate cortex, and other systems related to attention and short-term memory. Crystallized intelligence appears to be a function of brain regions that involve the storage and usage of long-term memories, such as the hippocampus. Research on training working memory and the training's indirect effect on fluid ability Because working memory is thought to influence gf, then training to increase the capacity of working memory could have a positive impact on gf. Some researchers, however, question whether the results of training interventions to enhance gf are long-lasting and transferable, especially when these techniques are used by healthy children and adults without cognitive deficiencies. A meta-analytical review published in 2012 concluded that "memory training programs appear to produce short-term, specific training effects that do not generalize." In a series of four individual experiments involving 70 participants (mean age of 25.6) from the University of Bern community, Jaeggi et al. found that, in comparison to a demographically matched control group, healthy young adults who practiced a demanding working memory task (dual n-back)