Summary: A new longitudinal study finds that different cognitive abilities appear to support each other during development, producing mutual gains in skills like reasoning and vocabulary and contributing to increases in general intelligence over time.
Source: APS.
Researchers have long observed that cognitive skills tend to be positively correlated: people who perform well on one type of test, such as reasoning, typically perform well on others, such as vocabulary. This consistent pattern allows researchers to summarize diverse test scores with a single factor often called “g” (general intelligence). Yet the processes that generate this positive manifold and shape the development of g are not fully understood.
“What this so-called ‘g-factor’ means is still very much up for debate,” says Rogier Kievit of the Cognition and Brain Sciences Unit at the University of Cambridge. “Is it a causal factor, an artefact of how tests are constructed, the result of education and environment, a consequence of genetics, an emergent property of interacting processes, or some combination of these?”
In new work led by Kievit with collaborators in Cambridge, London, and Berlin, researchers tested competing explanations for how cognitive abilities relate and change during late adolescence and early adulthood. The team analysed data from the Neuroscience in Psychiatry Network (NSPN), a Wellcome Trust–funded longitudinal cohort. The analysis included 785 participants aged 14 to 24, with 566 individuals tested on two occasions about 1.5 years apart. The investigators focused on two central cognitive domains that contribute to g: fluid reasoning (the ability to solve novel, abstract problems) and vocabulary (knowledge of word meanings).
The authors used statistical models designed to examine coupled developmental change — specifically, bivariate latent change score models — to compare three leading accounts: the g-factor theory (a single underlying factor driving all abilities), investment theory (where one ability is invested into another over development), and the mutualism model. The mutualism model proposes that cognitive abilities interact positively during development: stronger performance in one domain helps drive gains in another, and these reciprocal influences accumulate over time.
The study’s results supported the mutualism account. Participants with higher vocabulary scores tended to show greater gains in matrix reasoning over the follow-up interval, and participants with higher reasoning scores tended to show faster improvements in vocabulary. These reciprocal, dynamic pathways were the best fit to the longitudinal data and are not predicted by alternative accounts that treat abilities as independent or driven solely by a general latent factor.
These findings have important implications for understanding how individual differences in cognitive development emerge. Small advantages in a particular domain early in development can be amplified through mutualistic interactions, producing larger differences in overall cognitive ability later on. This process provides a plausible mechanism for how the positive manifold and the g-factor may arise through interacting developmental dynamics rather than from a single static source.
“Our findings may be relevant for early detection of developmental challenges,” Kievit notes. “Screening that focuses on single test outcomes can miss the broader pattern of interactions among domains. Tracking how abilities influence each other over time gives a richer and potentially more accurate picture of typical and atypical developmental trajectories.”
The study also speaks to longer-term outcomes. General cognitive ability is a strong predictor of many life outcomes, including academic and occupational success, health, and longevity. Understanding whether and how cognitive domains drive each other during development is therefore central to explaining why intelligence is so predictive of later functioning.
The authors emphasize that while the observed mutual associations are consistent with reciprocal developmental influences, the study does not definitively prove direct causation. Additional research is needed to identify the precise mechanisms that mediate these coupling effects and to determine how environmental factors, education, and biology contribute to the dynamic interplay among abilities.
Co-authors include Ulman Lindenberger (Max Planck Institute for Human Development); Ian Goodyer, Peter B. Jones, and Ed T. Bullmore (University of Cambridge); Peter Fonagy (University College London); the Neuroscience in Psychiatry Network (NSPN); and Raymond J. Dolan (Max Planck UCL Centre for Computational Psychiatry and Ageing Research).
Funding: The Neuroscience in Psychiatry Network is supported by a strategic award from the Wellcome Trust to the University of Cambridge and University College London (095844/Z/11/Z). R. A. Kievit is supported by the Wellcome Trust (Grant No. 107392/Z/15/Z) and the UK Medical Research Council (MCA060-5PR61). P. Fonagy is funded by a National Institute for Health Research Senior Investigator Award (NF-SI-0514-10157) and was partly supported by the NIHR Collaboration for Leadership in Applied Health Research and Care (CLAHRC) North Thames at Barts Health NHS Trust. The views expressed are those of the authors and do not necessarily reflect the views of the NHS, the NIHR, or the UK Department of Health.
Abstract
Mutualistic Coupling Between Vocabulary and Reasoning Supports Cognitive Development During Late Adolescence and Early Adulthood
One of the most replicable findings in psychology is the positive manifold: the observation that individual differences in cognitive abilities are universally positively correlated. Investigating the developmental origin of the positive manifold is crucial to understanding it. In a large longitudinal cohort of adolescents and young adults (N = 785; n = 566 across two waves, mean interval between waves = 1.48 years; age range = 14–25 years), the authors examined changes in fluid reasoning and vocabulary. Using bivariate latent change score models to compare g-factor theory, investment theory, and mutualism, they found that a mutualism model — where basic cognitive abilities directly and positively interact during development — provided the best account of developmental changes. Individuals with higher vocabulary scores showed greater gains in matrix reasoning and vice versa. These dynamic coupling pathways offer a mechanistic perspective on cognitive development.
Original research: “Mutualistic Coupling Between Vocabulary and Reasoning Supports Cognitive Development During Late Adolescence and Early Adulthood” by Rogier A. Kievit, Ulman Lindenberger, Ian M. Goodyer, Peter B. Jones, Peter Fonagy, Edward T. Bullmore, the Neuroscience in Psychiatry Network, and Raymond J. Dolan, published in Psychological Science, 2017. DOI: 10.1177/0956797617710785.