(This is a bit brief given the complexity of the question. I'll expand later if I can summarize more of the findings.)
Probably the only thing which is certain is the high-failure rate in CS1-level classes, worldwide. From a recent (2014) meta-analysis:
In this paper, we answer the call for further substantial evidence on the CS1 failure rate phenomenon, by performing a systematic review of introductory programming literature, and a statistical analysis on pass rate data extracted from relevant articles. Pass rates describing the outcomes of 161 CS1 courses that ran in 15 different countries, across 51 institutions were extracted and analysed. An almost identical mean worldwide pass rate of 67.7% was found. Moderator analysis revealed significant, but perhaps not substantial differences in pass rates based upon: grade level, country, and class size. However, pass rates were found not to have significantly differed over time, or based upon the programming language taught in the course.
There was a prior world-wide study in 2007 (using online self-reported data) which agreed with this number (~33% failure rate), although the distribution observed for the passing students was different:
Alas neither paper compares this number with other fields, but the 2007 one also calculated that graduation from CS tertiary education is low worldwide, 26.8% so getting past CS1 hardly guarantees smooth sailing thereafter.
The latter [counterproductively 3D] graph is based on UNESCO data, which is why North America is absent (the US boycott[ed] UNESCO.)
There is however one paper which does state that:
Studies in Ireland  have shown that Computer Science has the highest
failure rate, giving a 26.9% non-completion rate for Computing
 Mark Morgan, Rita Flanagan and Thomas Kellaghan, "Higher Education Report A Study of Non-Completion in Undergraduate University Courses" published February 2001
Several theories for the CS1 failure rate have been advanced: genetic basis, "learning edge momentum", and/or stumbling blocks but it's too early to tell if any of these have convincing empirical support. (In fact the last paper argues that insofar there isn't a good data gathering strategy that would even allow one to one to decide.) And one academic blog suggests (based on its own data) that none might be correct. Yet another paper suggests that (Duckworth's) grit plays a substantial role. And with respect to the final grade being an aggregation one paper (based on a single course) suggests that students' bimodal final grades come from the practical side of the evaluation:
Alas I was not able to find other papers looking into this aspect, however it does look to me that the grit paper does point in the same direction; it found that grit was a much better predictor than ACT (itself considered a proxy for IQ) for CS1 success.
The humps paper mentiond in the question is not taken very seriously (most of these papers dont' cite it), and even by its own authors themselves these days. It also does not seem to have been the first to observe the grade distribution phenomenon. I'm not entirely sure why it became popular on-line. Its strong language and/or the SE-effect maybe. For instance this 2005 paper noted the 30% to 40% failure rate as well.
Also here's a graph with the failure rate by country from the 2014 meta-analysis:
The authors note that the top 3 failure leaders Portugal, Germany and Brazil were represented by a pretty small sample in this study, so the data for those countries cannot really be relied upon. On the other hand, "of the 4 countries
which made 80% of the sample, Finland was found to have the lowest pass rate 57.7%". The other three countries in this 80% bulk of the data, US, UK, and Australia had statistically indistinguishable rates.
Also, the methodology should have been more upfront in this meta-analysis but they do say "Currently we have found in this study 3/10 students do not complete, or fail CS1." So they definitely count withdrawals as failure, basically, for the statics they reported.