What major assumption of the Big Five model was called into question by the findings of Van der Linden et al. (2010)?

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Van der Linden et al. (2010) found evidence that the Big Five factors are not completely independent, as previously assumed. More specifically, they found significant correlations between the Agreeableness and Conscientiousness factors, as well as between the Neuroticism and Extraversion factors, which suggests that these factors may be more closely related than previously thought. This calls into question the major assumption that the Big Five factors are all independent.

What Major Assumption of the Big Five Model Was Called