Integration/differentiation of mental processes is major mechanism of development. Developmental theories ascribe intellectual development to it. In psychometric theory, Spearman's law of diminishing returns postulates that increasing g allows increasing differentiation of cognitive abilities, because increased mental power allows variable investment in domain-specific learning. Empirical evidence has been inconsistent so far, with some studies supporting and others contradicting this mechanism. This state of affairs is due to a developmental phenomenon: Both differentiation and strengthening of relations between specific processes and g may happen but these changes are phase-specific and ability-specific, depending upon the developmental priorities in the formation of g in each phase. We present eight studies covering the age span from 4 to 85 years in support of this phenomenon. Using new powerful modeling methods we showed that differentiation and binding of mental processes in g occurs in cycles. Specific processes intertwine with g at the beginning of cycles when they are integrated into it; when well established, these processes may vary with increasing g, reflecting its higher flexibility. Representational knowledge, inductive inference and awareness of it, and grasp of logical constraints framing inference are the major markers of g, first intertwining with in their respective cycles and differentiating later during the periods of 2-6, 7-11, and 11-20 years, respectively. The implications of these findings for an overarching cognitive developmental/differential theory of human mind are discussed.

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This study investigated developmental differences in some metacognitive variables in illdefined problem solving and their possible connections with cognitive development in adulthood. Participants were 57 individuals of different ages (adolescents, young adults, mature adults and older adults). They solved one well-defined and six ill-defined problems while their thinking-aloud was taped. They then answered a metacognitive statements questionnaire. Differences in performance were statistically significant in all problems: the best results in interpolation and divergent production problems were achieved by the younger adult group and the best performance on most dialectical everyday problems was found in the mature adults' group. We found no significant differences between age groups in the on-line monitoring of the solving process. Accuracy in metacognitive statements was however significantly better in the mature adult and the younger adult groups. Awareness of and reflection on one's own mental processes showed a similar developmental pattern to relativistic/dialectical thought: low expression in adolescence, an increase in early adulthood, a peak in mature adulthood and a minor decline in later years.

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Cognitive deficits are a potential part of the clinical picture of depression, especially when it comes to late-life depression. The present study was carried out to establish whether distinctive cognitive deficiencies can be linked with depression in the late-life period, especially in executive functioning, working memory, and visuospatial memory. Our sample consisted of 71 seniors in the age range between 69 and 85 years. A battery of neurocognitive tests was used, including tests of executive functioning (Trail Making Test [TMT], part B, Stroop color word test, semantic word fluency test, and partially Rey-Osterrieth Complex Figure test [ROCF]), tests of attention and working memory (TMT, part A and digit span), and test of visuospatil ability and memory (ROCF). Results demonstrated that depression scores were significantly negatively correlated with scores on Verbal Fluency test, Stroop test, and ROCF immediate copy and recall. Depression was also linked to slower functioning on Trail Making A and Trail Making B subtest. In general, higher depression scores were correlated with lower performance on neuropsychological tests. However, digit span showed no significant correlation with depression. In addition, results of regression analyses revealed that the strongest predictors of depression were performance on the Rey-Osterrieth test-immediate recall and Stroop test. Thus, we could ascertain that difficulties in executive functioning and visuospatial memory are the best predictors of depression in elderly.