Ed Zigler's legacy and the developmental approach to the study of persons with intellectual disability

This special issue is a brief testimony, of sorts, to Ed Zigler's impact on the field of intellectual disability (ID) beginning with his initial contributions in the 1960s. In contrast to the ‘deficit’ approach to ID that dominated this area of study for decades, Ed's ‘developmental approach’ was an inclusive one, in that it placed the development of persons with ID in the same realm as typically developing children (Burack et al. 2021a). According to Ed, the development of typically and atypically developing children was similarly impacted by personality, motivational and other personal, familial and contextual factors. Ed's focus was on those persons with ID whose low IQ resulted primarily from the polygenetic transmission of intelligence from their parents (Ed referred to this as familial ID) who happen to have simply fallen along the low end of the normal IQ distribution (Zigler 1967, 1969; Zigler & Hodapp 1986; Hodapp 2021). However, Ed, along with his students and colleagues explored the extent to which developmental principles also applied to persons with any one of the myriad neural or genetic aetiologies of ID (e.g., Weisz & Zigler 1979; Hodapp et al. 1990; Hodapp & Zigler 1990) and ultimately argued for the necessity of differentiating among persons with ID by aetiology in order to identify clear developmental patterns in these diverse conditions (Burack et al. 1988, 1990, 2012b, 2021b). The articles in this volume are testament to that type of precision in the study of developmental patterns among persons with ID that can only be attained by the study of specific aetiological groups. In their examination of extremely low birthweight (ELBW) survivors, Mathewson et al. introduce a population that is not often considered in publications of persons with ID despite the implications for Ed's developmental approach. Mathewson et al. report that the children with borderline intellectual functioning had significantly lower birthweights than the ELBW survivors without borderline intellectual functioning and were more likely to experience neurosensory impairment and other complications. Consistent with the developmental approach and Ed's emphasis on the various contextual factors that can mediate the development of children with ID, those with borderline intellectual functioning were more likely to be born of mothers of low socioeconomic status, which in turn was associated with lower academic scores. The remainder of the empirical papers are focused on developmental considerations of children with a specific identifiable genetic aetiology. Consistent with Ed's emphasis on the centrality of matching by appropriate indexes of mental age in comparisons of persons with and without ID, Matusuba et al. find similar patterns of performance between participants with Down syndrome (DS) and matched typically developing children on a task of the filtering component of attention. In their study of participants with 3q29 syndrome – a (typically de novo) mutation associated with ID – Klaiman et al. invoke another core aspect of Ed's developmental approach: understanding cognitive profiles that speaks to the processes of differentiation and integration in development. In another example illustrating the importance of studying profiles across domains, Onnivello et al. report that infants with DS exhibit strengths in expressive, relative to receptive, communication. At first glance, this finding is at odds with much of the literature on DS noting relative strengths in receptive language. However, Onnivello et al. highlight the importance of studying infants, as well as the importance of differentiating language from communication, and offer important implications for early intervention. In their other contribution to this special issue, Onnivello et al. highlight how understanding sequences and milestones of early cognitive and communication development underscores the need for a more individualised approach to assessment and intervention for infants with DS. In another intervention-related study, Dimachkie-Nunnally et al. examine the profiles and trajectories of executive functioning (EF) in young children with DS and perform a waitlist-control intervention to improve language. They report relative strengths in flexibility and inhibitory self-control, but a relative weakness in metacognition. The language intervention group showed positive, albeit temporary, effects on EF skills, highlighting the link between various EF skills and language/communication. In the one review paper in this special issue, Burack et al. revisit ‘the mysterious myth of attention deficit’ in the way of a systematic review with two critical criteria that emanated from Ed's developmental approach: (1) the importance of studying homogenous populations that reflect a well-characterised specific aetiology and (2) appropriate developmental age-matching strategies. Consistent with the conclusions of the two previous iterations of the mysterious myth formulation (Burack et al. 2001, 2012a), the findings of this systematic review contradict longstanding beliefs about attention-based deficits among persons with ID, which were largely based in faulty methodologies in which essential developmental considerations are ignored. In reflecting on these contributions, we highlight in the biographical piece at the end of this issue that anyone who worked with Ed drew their own sense of passion and commitment from him, and as a result, his impact had guided generations of scholars in the study of persons with ID. This special issue represents a modest and cursory overview of the ways in which Ed's work continues to impact the field. We think Ed would be gratified to know that the field has advanced to the extent that it has and hope that, in some small way, this special issue represents some of those advances and speaks to Ed's lasting influence on the field and on the subsequent generations of researchers who are dedicated to improving the lives of persons with ID.