Mortgage Risk Weights – revisited

I post on a range of topics in banking but residential mortgage risk weights is one that seems to generate the most attention. I first posted on the topic back in Sep 2018 and have revisited the topic a few times (Dec 2018, June 2019#1, June 2019#2, and Nov 2019) .

The posts have tended to generate a reasonable number of views but limited direct engagement with the arguments I have advanced. Persistence pays off however because the last post did get some specific and very useful feedback on the points I had raised to argue that the difference in capital requirements between IRB and Standardised Banks was not as big as it was claimed to be.

My posts were a response to the discussion of this topic I observed in the financial press which just focussed on the nominal difference in the risk weights (i.e. 25% versus 39%) without any of the qualifications. I identified 5 problems with the simplistic comparison cited in the popular press and by some regulators:

• Problem 1 – Capital adequacy ratios differ
• Problem 2 – You have to include capital deductions
• Problem 3 – The standardised risk weights for residential mortgages seems set to change
• Problem 4 – The risk of a mortgage depends on the portfolio not the individual loan
• Problem 5 – You have to include the capital required for Interest Rate Risk in the Banking Book

With the benefit of hindsight and the feedback I have received, I would concede that I have probably paid insufficient attention to the disparity between risk weights (RW) at the higher quality end of the mortgage risk spectrum. IRB banks can be seen to writing a substantial share of their loan book at very low RWs (circa 6%) whereas the best case scenario for standardised banks is a 20% RW. The IRB banks are constrained by the requirement that their average RW should be at least 25% and I thought that this RW Floor was sufficient to just focus on the comparison of average RW. I also thought that the revisions to the standardised approach that introduced the 20% RW might make more of a difference. Now I am not so sure. I need to do a bit more work to resolve the question so for the moment I just want to go on record with this being an issue that needs more thought than I have given it to date.

Regarding the other 4 issues that I identified in my first post, I stand by them for the most part. That does not mean I am right of course but I will briefly recap on my arguments, some of the push back that I have received and areas where we may have to just agree to disagree.

Target capital adequacy ratios differ materially. The big IRB banks are targeting CET1 ratios based on the 10.5% Unquestionably Strong Benchmark and will typically have a bit of a buffer over that threshold. Smaller banks like Bendigo and Suncorp appear to operate with much lower CET1 targets (8.5 to 9.0%). This does not completely offset the nominal RW difference (25 versus 39%) but it is material (circa 20% difference) in my opinion so it seem fair to me that the discussion include this fact. I have to say that not all of my correspondents accepted this argument so it seems that we will have to agree to disagree.

You have to include capital deductions. In particular, the IRB banks are required to hold CET1 capital for the shortfall between their loan loss provision and a regulatory capital value called “Regulatory Expected Loss”. There did not appear to be a great awareness of this requirement and a tendency to dismiss it but my understanding is that it can increase the effective capital requirement by 10-12% which corresponds to an effective IRB RW closer to 28% than 25%.

The risk of a mortgage depends on the overall portfolio not the individual loan. My point here has been that small banks will typically be less diversified than big banks and so that justifies a difference in the capital requirements. I have come to recognise that the difference in portfolio risk may be accentuated to the extent that capital requirements applied to standardised banks impede their ability to capture a fair share of the higher quality end of the residential mortgage book. So I think my core point stands but there is more work to do here to fully understand this aspect of the residential mortgage capital requirements. In particular, I would love get more insight into how APRA thought about this issue when it was calibrating the IRB and standardised capital requirements. If they have spelled out their position somewhere, I have not been able to locate it.

You have to include the capital required for Interest Rate Risk in the Banking Book (IRRBB). I did not attempt to quantify how significant this was but simply argued that it was a requirement that IRB banks faced that standardised banks did not and hence it did reduce the benefit of lower RW. The push back I received was that the IRRBB capital requirement was solely a function of IRB banks “punting” their capital and hence completely unrelated to their residential mortgage loans. I doubt that I will resolve this question here and I do concede that the way in which banks choose to invest their capital has an impact on the size of the IRRBB capital requirement. That said, a bank has to hold capital to underwrite the risk in its residential mortgage book and, all other thinks being equal, an IRB bank has to hold more capital for the IRRBB requirement flowing from the capital that it invests on behalf of the residential mortgage book. So it still seems intuitively reasonable to me to make the connection. Other people clearly disagree so we may have to agree to disagree on this aspect.

Summing up, I had never intended to say that there was no difference in capital requirements. My point was simply that the difference is not as big as is claimed and I was yet to see any analysis that considered all of the issues relevant to properly understand what the net difference in capital requirements is. The issue of how to achieve a more level playing field between IRB and Standardised Banks is of course about much more than differences in capital requirements but it is an important question and one that should be based on a firmer set of facts that a simplistic comparison of the 39% standardised versus 25% IRB RW that is regularly thrown around in the discussion of this question.

I hope I have given a fair representation here of the counter arguments people have raised against my original thesis but apologies in advance if I have not. My understanding of the issues has definitely been improved by the challenges posted on the blog so thanks to everyone who took the time to engage.

Tony