I found this in a slide presentation by Malcolm Gammie QC, one of the directors of the Institute for Fiscal Studies Mirrlees review, when talking about his work in that capacity:
What are relevant criteria for identifying taxable capacity? - earnings, expenditure, age, hair, status, education, gender, height, assets, disability, responsibilities?
My guess is he thinks that funny.
Many of us might think it a pretty sick sense of humour, especially given the discrimination all too readily apparent in the work the review has commissioned. Notice he forgot to mention wealth. Why? Was he trying to tell us something?