Scientists have long agreed that human development reflects a complex interplay between inherited traits and the environments in which people are raised. How much each factor contributes remains a central question in psychology and developmental science.
To shed light on the nature-versus-nurture debate, UC Santa Barbara psychologist John Protzko reanalyzed data from a major longitudinal trial to determine whether intensive early-life environmental interventions produced lasting changes to children’s intelligence—particularly among those born at low birth weight.
Protzko’s central finding: targeted interventions did increase measures of intelligence in early childhood, but those gains were not permanent. Once the interventions ended, their effects diminished over time, a pattern psychologists call the “fadeout effect.” The work is reported in the journal Intelligence.
“Certain environmental interventions can raise general intelligence,” said Protzko, a postdoctoral scholar in the META (Memory, Emotion, Thought, Awareness) Lab in UCSB’s Department of Psychological & Brain Sciences. “This is not merely an upward shift in test scores; the interventions appear to produce changes in underlying cognitive ability. Still, those improvements tend to fade once the intensive support stops.” Researchers distinguish between IQ test scores, which quantify performance at a given time, and general intelligence (g), which reflects an underlying, latent cognitive factor.
Protzko reexamined outcomes from the Infant Health and Development Program (IHDP), a randomized controlled trial that followed 985 children born at low birth weight. During the first three years of life, participating children received a rigorous, cognitively demanding program designed to offset early biological risk. The intervention combined educational, medical, and family-support elements intended to boost early cognitive development.
Researchers assessed the children’s abilities using the Stanford-Binet Intelligence Scales at age three to establish a baseline. Follow-up testing occurred at ages five and eight—at least two years after the intervention had ended—to evaluate whether any cognitive gains persisted.
At age three, children who received the intervention showed measurable increases in general intelligence compared with control groups. However, by age five the effect had largely disappeared, and similar null findings were observed at age eight. Protzko’s reanalysis demonstrates that the fadeout phenomenon can apply directly to the latent construct of general intelligence—not only to surface-level test performance.
Beyond documenting fadeout, the study raises important questions about causality. One common interpretation of correlations in developmental data is that intelligence at one age causally influences intelligence at later ages. Protzko’s findings complicate that view.
“My analysis suggests intelligence measured in early childhood may not operate as a straightforward causal driver of later outcomes, at least in young children,” Protzko explained. “If an increase in early intelligence does not persist after an intervention ends, it calls into question simple causal narratives. More work—especially studies that extend into adulthood—is needed to fully untangle causality.”
This paper is the second in a series by Protzko examining fadeout. Both papers emphasize a unidirectional reaction model: intelligence can flexibly adapt upward when environmental demands increase, but it tends to return toward a prior level when those demands are removed. In other words, gains may reflect adaptive responses to a richer environment rather than permanent rewiring of cognitive capacity.
“Raising IQ through an intervention is not merely a superficial change in test performance,” Protzko said. “Targeted environmental supports can elevate both IQ scores and the underlying g factor, but these gains often diminish once intensive supports stop.”
Importantly, the analysis does not argue that early interventions are futile. On the contrary, Protzko emphasizes that such programs can still provide meaningful short-term benefits and may alter developmental trajectories in ways not fully captured by IQ alone.
“I believe it remains valuable to intervene early and try to change the life course for children at risk,” he said. “Understanding fadeout helps us design better interventions and follow-up strategies to sustain benefits over the long term.”
Source: Julie Cohen – UCSB
Image Source: Image adapted from the UCSB press release.
Original Research: John Protzko, “Does the raising IQ–raising g distinction explain the fadeout effect?” Intelligence. Published online March 12, 2016. doi:10.1016/j.intell.2016.02.008
Abstract
Does the raising IQ–raising g distinction explain the fadeout effect?
Previous studies show that the benefits of IQ-boosting interventions often fade after the program ends. This paper examines whether fadeout occurs because interventions raise test performance without changing the underlying general intelligence factor g. Reanalyzing a large randomized controlled trial (N = 985) that provided intensive intervention from birth through age three, the study tested whether the intervention increased the latent g factor under strict measurement invariance. Results indicate the intervention did raise g at age three, but no effects remained at follow-up assessments at ages five and eight. Thus, the distinction between raising IQ and raising g does not fully explain fadeout: environmental improvements to g can still dissipate when supports are withdrawn.