Developmental concept of idiocy.
The age-of-onset rule for ID is a historical leftover, not a scientific must—so keep your diagnostic lens flexible.
01Research in Context
What this study did
Murray looked at the history of the rule that says intellectual disability must start in childhood.
He traced how this rule first appeared in 1800s laws, not in science.
The paper is a deep dive into old legal texts and medical writings.
What they found
The developmental-onset rule is a leftover from 19th-century politics, not a medical fact.
Doctors kept the rule because lawmakers needed it, not because data proved it.
This means the age-of-onset rule can be questioned today.
How this fits with other research
Smith (2007) also digs into history but warns against judging past figures by today's rules.
Danforth et al. (2010) shows how labels like "educable" were shaped by social stories, just like the age rule.
Porter et al. (2008) argues that economic forces now shape ID services, echoing how legal forces once shaped diagnosis.
Together these papers show that ID criteria are always tied to the times we live in.
Why it matters
You can now ask: does this client truly need to show childhood delays, or is that just an old habit? When a teen has a brain injury and drops 30 IQ points, Murray gives you room to call it ID even without early records. This opens doors to services and rights that rigid age rules would block.
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02At a glance
03Original abstract
In dominant definitions of mental retardation, researchers have insisted on the diagnosis being restricted to conditions manifested during the developmental period. However, even in the 19th century, this was only one of several conceptual options, some of which did not exclude adult brain injury or dementia. Events in the 19th and early 20th centuries, particularly with the growth of institutions, scientific study, and, later, intelligence testing led to these other options being excluded. Here, I discuss the proposal that current definitions are highly contingent on factors that are neither essential nor necessary. Although not arguing for any specific changes to current definitions, I do argue that theoretical options should be kept open and that dominant ones should be questioned.
Intellectual and developmental disabilities, 2007 · doi:10.1352/1934-9556(2007)45[23:DCOI]2.0.CO;2