Which of the following is a requirement for the diagnosis of Intellectual Developmental Disorder (IDD)?

For a diagnosis of Intellectual Developmental Disorder (IDD), the requirement is that the onset must occur during the developmental period. This is essential because IDD is characterized by limitations in both intellectual functioning and adaptive behavior that emerge before the age of 18. Recognizing that these issues manifest in childhood is critical to differentiate IDD from other disorders that may develop later in life, such as neurocognitive disorders. This timing is not only crucial for accurate diagnosis but also for understanding the impact on an individual's development and needed interventions.

In contrast, symptoms arising in adulthood would be indicative of a different disorder, as IDD is defined explicitly by its developmental onset. Evidence of severe brain injury or the inheritance of a genetic disorder may be associated factors in some cases of IDD but are not prerequisites for a diagnosis, as IDD can arise from a variety of causes. Therefore, the defining characteristic is the emergence of symptoms during the developmental stages of life.