I think most people would be satisfied with the description of autism as a spectral condition epitomised by the phrase 'if you've met one person with autism, you've met one person with autism'. The term 'autism spectrum' denotes the heterogeneity apparent and that the presentation of core symptoms varies intra-diagnosis according to symptom domain as well as inter-person across different individuals. This is independent of the fluctuations in presentation across different situations and not including variables like maturation, co-morbidity and any effect from intervention.
A spectrum also implies other things including degrees of severity between two poles; ranging from very severe 'disability' at one end to 'difference' at the other end (as per this description from Lorna Wing) depending on your definition and how you grade severity. In between you have a mix of ability and disability. I should point out that by using the term 'difference' I am not in anyway suggesting that those at that end of the spectrum are in any way less deserving of the appropriate support and services. High functioning - however you describe this - does not always mean 'can function'.
At the less severe end of the spectrum, a cut-off point is generally accepted to exist in some shape or form; in diagnostic terms denoting presentation which somehow surpasses a clinical threshold and in cold hard psychiatric terms implies psychopathology. The rights and wrongs of where the threshold has been currently set still continues to create lively debate. The presence of that diagnostic threshold however does not necessarily denote the 'end of the autism spectrum' from a real-world behavioural perspective. It implies the end of the diagnostic spectrum but not that the subtle presentation of certain autistic traits cannot be present beyond the clinical spectrum. Enter the broader autism phenotype (BAP) as evidence of this extra-diagnostic spectrum of presentation.
I first heard about the BAP quite early on in my research career. Prof. Ann Le Couteur, one of the early proponents of the BAP discussed some of the ins and outs of the concept and in particular based on studies of siblings of children with autism, how issues like speech and language problems and social interactive issues were picked up from time to time which did not quite fulfil autism diagnostic criteria either in measure or severity. I also remember quite a few people talking about the overlap with certain personality traits such as introversion and more recently some work looking at autistic traits in conditions such as feeding disorders.
Much of the early work on BAP was tied into the genetic basis of autism based on twin studies, the strength of which has recently come under scrutiny. With this in mind, a recent paper by Davidson and colleagues* caught my eye with their suggestion ".. that BAP traits occur at low rates in simplex families". The results reported in this recent paper were based on an examination of the Simons Simplex Collection which as the name suggests contains details of samples from around 2700 families with one child diagnosed with an autism spectrum condition. Davidson et al looked at over 1500 of these families with the purpose of trying to further elucidate what the BAP is and how it could be appropriately tested for and measured.
I'm intrigued about the potential for differences in BAP traits according to whether a family has one child with autism or more than one. The obvious issue with this study is its exclusive analysis of simplex families, so nothing to compare against in terms of multiplex families outside of external datasets. That and the reliance on a snapshot of where a family is in terms of autism not necessarily ruling out any elevated risk of autism recurrence in simplex families should for example they have other children. This last point is perhaps a fundamental flaw in any simplex analysis.
Putting these issues aside, the first thing that comes to mind is whether the weighting of genetics vs. environment might be different in simplex families where BAP traits are infrequent. In other words, is there a suggestion that environmental factors might trump genetic factors in such cases compared to other families where BAP traits are more frequent in other family members? It is perhaps not as easy to say one is genetic and the other is environment because such simple arguments have not been borne out by the research data and most (if not all) conditions are likely to be the result of an interplay between nature and nurture. Think epigenetics for example.
If the data from Davidson is accurate however, this might provide a good opportunity to look at factors such as regression, comorbidity and early adverse events to determine any difference in individuals with autism among high and low BAP trait families. At the very least if offers another potential phenotypic distinction which could be added to those already being looked at.
* Davidson J. et al. Expression of the Broad Autism Phenotype in Simplex Autism Families from the Simons Simplex Collection. JADD. March 2012.