A final reply to te Nijenhuis et al. (2019)

Area of Science:

• Cognitive psychology
• Behavioral genetics

Background:

• The Jensen effect describes how group differences in cognitive abilities widen with task complexity.
• Distinguishing genetic from environmental influences on IQ is complex, especially with educational interventions.

Purpose of the Study:

• To investigate whether the presence of a Jensen effect or an anti-Jensen effect can reliably differentiate genetic from environmental causes of group differences in IQ.
• To determine if these effects are sufficient indicators for attributing causality.

Main Methods:

• Analyzing cognitive task performance in groups before and after educational training.
• Examining IQ test performance on items of varying difficulty levels.
• Evaluating the relationship between group differences and task complexity (Jensen effect).

Main Results:

• The presence of a Jensen effect (groups differing more on complex tasks) does not definitively indicate genetic causes.
• An anti-Jensen effect (groups differing more on simpler tasks) does not definitively indicate environmental causes.
• The interpretation of Jensen effects requires careful case-by-case analysis.

Conclusions:

• Jensen effects and anti-Jensen effects are not sufficient on their own to determine whether group differences in IQ are genetic or environmental.
• Causality must be assessed individually for each specific case, considering multiple factors beyond task complexity.
• Relying solely on Jensen effects for genetic vs. environmental attributions is scientifically unsound.