Primary vs. Secondary Deficits for Newbies: What are they and why do they matter?

One of my goals on this blog is to bridge the gap between working researchers and laypeople who use scientific research to make decisions and understand themselves better.  As my real-life friends and family have pointed out after reading, I don't always succeed.  While I usually remember to explain psych and brain jargon, it hasn't occurred to me to explain the unique style of thinking cognitive researchers use or the sorts of questions we ask.  But we really do have our own ways of thinking about problems, just like physicists or economists or historians or practitioners of any other discipline.  Knowing these ways of thinking can make the research studies we produce a lot more understandable.

For one, we see the mind as something that has a lot of parts that do a lot of things, some of which are necessary for doing other things.  Pick any task--speaking, looking, reading, paying attention, recalling a fact, paying attention--and you'll find it's made up of many subprocesses.  We usually only see what exact subprocesses there are when something goes wrong, such as when a person gets brain damage.  Brain damaged patients helped us understand that understanding and producing language are separate functions because some groups of patients could understand but not produce, and others had the opposite impairment.*  Most of the time we can't see where the "joints" are in a function because brain damage isn't usually so neat and specific, but we know they're there, and we try to design experiments to find out where they are.

(A lot of really arcane debates on experimental methodology exist because we're really asking, "is this experiment designed in such a way that it reveals the 'joints' underlying a particular ability?"  You have to be extremely clever, detail-oriented, and good at methodology and statistics to do this well, but it can be hard for a non-expert to follow).

That brings me to a major distinction cognitive/neuroscience people** like to make that I may have even used on this blog: primary versus secondary deficit.

A primary deficit is the first deficit a person starts out with.  It is "primary" causally, genetically, neurologically, behaviorally, conceptually, and so on.  A secondary deficit is one that develops out of the primary deficit.  The idea here is that the primary deficit derails a person's development, causing more problems to develop.  Research on developmental disabilities is full of debates on which deficits are primary and which secondary because without knowing this, we can't trace out the causal pathways by which disorders develop and without knowing how they develop, we won't be able to time interventions effectively or come up with effective treatments.

To make this more concrete: ADHD involves real-world impairments in attention, so one might think that an attention deficit is primary.  A person with ADHD has genes that disrupt their ability to pay attention in certain ways, this problem develops early in life, and it leads to other problems like poor self-control or academic difficulties.  However, most ADHD researchers argue that the attention deficit is actually secondary--it results from a more basic impairment in something else.  They do not agree on what that the primary cause(s) are.  One candidate is "delay aversion," a disruption in time perception that makes a wait seem longer and more upsetting to a person with ADHD than it would to the average person.  Along similar lines, there may be a disruption in how people with ADHD respond to reward, which may be mediated neurochemically by the dopamine system.  Another possibility is a general disruption in executive function.  Lastly, in at least some people with ADHD, there may be a disruption in physiological arousal--basic alertness and ability to respond to things in the environment--which interferes with a person's functioning in other areas.  All of these theories say that these disruptions have a number of effects, including interfering with a person's ability to pay attention, thus causing the real-world symptoms of ADHD.

So, you can see that secondary deficits are very real--calling something a secondary deficit does not mean writing it off--but they are not considered to be at the "root" of the disorder.  Attention problems exist and make life difficult for people with ADHD, but if they're secondary, ADHD can't properly be called an "attention disorder."

Why does this matter?  Consider autism, where sensory and motor disabilities coexist with social and communication ones.  One can ask: are the sensory-motor disabilities primary, or are the social-communication ones primary, or both?  Indeed, researchers have recently started debating exactly this question, and much of this blog is devoted to arguing for one answer (that sensory-motor disabilities are primary).  People who consider sensory-motor disabilities to be primary make two basic arguments: 1) sensory and motor deficits appear in infancy months before social and communication ones; thus, they could conceivably cause social-communication ones but not vice versa; 2) social and communication skills depend on certain sensory and motor abilities, so sensory-motor disruption will often entail social-communication disabilities.  People who consider social-communication disabilities to be primary appeal to the DSM--social and communication disabilities are central to the autism diagnosis and unique to it, whereas most developmental disabilities involve some sort of sensory and/or motor disruptions.

If sensory-motor disruptions are primary, then the earliest interventions should be targeted at helping to remediate or improve these skills.  Any social and communication problems that later develop anyway can then be treated later.  However, most interventions aren't like this.  They target social and communication symptoms from the beginning, and a child is lucky to have any sensory or motor intervention at all.

There's also the question of how we look at people with autism, and how they see themselves.  First, people who see social-communication disabilities as primary are privileging an outside perspective of autism--how their behavior appears to neurotypicals--a perspective that by the very nature of their disability, autistics don't have.  However, they're choosing to push aside as secondary the experiences autistics live with every day and often write about eloquently.  Only the most radical of autistic adults would deny that their social-communication disabilities exist, but they argue that these are secondary to extreme sensory and motor differences from the norm.  For researchers to hypothesize that sensory-motor disabilities are primary, then, is to take the perspective of autistics seriously.  Thus there is an ethical issue here as well as a factual and scientific one.

Second, a sensory-motor disability evokes very different attitudes than a social-communication disability.  Unless you're careful how you define things, a primary social-communication disability shades awfully close to things like lack of empathy or indifference to others.  When we believe these things about other people, we see them as less human and less capable of having fulfilling relationships, and when autistics believe these things about themselves, they start to see themselves this way.  A sensory-motor disability evokes more of a sense of a person trapped inside a severely limited body (in the case of motor disabilities) or lacking information and experiences the rest of us share (in the case of sensory disabilities).  While these perceptions can lead some to pity disabled people, it doesn't make us view them as a fundamentally different species.  Just consider the difference between how blind and deaf people are presented in the media versus people with autism or Aspergers.  Both are problematic, as any disabilities activist will tell you, but the presentation of those with autism is much more dehumanizing.

In short, whether a deficit is primary or secondary matters--not just intellectually, but also practically and ethically.  That's why the question of which deficit is primary is so hotly debated in developmental disabilities research.  Watch out for this question next time you read a news article or a scientific paper.

*A case like this, where there are 2 functions A and B and one group has impairment in A but not B and the other has an impairment iin B but not A, is called a double dissociation.  You need double dissociation to show that two subprocesses really are separate.  If one group has impairment in A but not B and the other has impairment in both, it's possible that A is just a harder task for people and therefore more likely to suffer impairment.
**I've lumped together several disciplines with different names that take the same basic approach--cognitive psychologists, cognitive neuroscientists, and cognitive scientists.  We're found in departments with all sorts of different names, but we share this way of thinking and an interest in how people think and perceive.  Until recently, most of us ignored emotions and personality, even though they clearly interact with cognition and perception.  So someone like John Gottman--a favorite of mine--is not a cognitive researcher.


  1. >>>Indeed, researchers have recently started debating exactly this question, and much of this blog is devoted to arguing for one answer (that sensory-motor disabilities are primary). <<<

    I will have to start reading your blog more.

    1. Please do. I love your comments. Still mulling over what you said about Benno's experiences...

      I find the more I read sensory and motor research the harder it is to post about it, because so many skills are being researched in so many ways that it's hard to figure out how all the findings fit together into a larger picture. Definitely something I'm coming back to, though, so stay tuned. :)