Preregistration Abstract: Although intelligence and mental health have been recognized as the two core pillars of individual adaptation, growth, and opportunity, little is known about their interrelation. Here we chart the developmental interplay between the p-factor of psychopathology, which indicates a general vulnerability to experience psychopathology symptoms, and the g-factor of general intelligence that describes the ability to think, reason, and learn in approximately 4,600 twin pairs from England and Wales, who were followed up at the ages 7, 9, 12, and 16 years. Using cross-lagged twin models, we expect to find bidirectional effects, with p- and g-factors influencing each other over time, that are overall modest in size. We will also explore if these cross-lagged effects are due to genetic or environmental influences. These findings could point to an even broader tendency for experiencing unfavourable developmental outcomes that sits atop p- and g-factors.