How about 'length of hair'? Getting your hair cut (mens and womens) can be expensive, could be considered a 'luxury' and so may have been cut back on (ahem!), yielding, on average, longer hair. Might need to be 'seasonally adjusted' and might be influenced by people, like me in fact, swapping a trip to the barber for a set of clippers and trust in your wife to do it!