Was reading a study done for the EASA back in 2009 which just about agreed with what you came up with Mach E Avelli, what was also noted was that the average person weighed about 5kg more during the Winter than the Summer, I think this was mostly put down to the type of clothing being worn but may have had some kind of biological reasoning for it as well.
http://www.easa.europa.eu/rulemaking...95%20Final.pdf
Thats the link, well worth a read, especially for those using the 77kg. The average male it was found in the above survey weighed 88.7kg in Summer and 93.5kg in Winter with an average handbag weight of 6kg in Summer and 7.2kg in Winter, a heck of a lot more than 77kg, especially if you're talking about a fully loaded aircraft with 100+ seats!!! This survey was conducted on just over 22,000 Passengers so its a pretty wide range.
SOME OTHER FUN FACTS - Yes I'm this bored
The average weight for a males "Handbag" was slightly more than a females...go figure?! Could have just been a lot of blokes minding the missus bag though
Though you'll be glad to know that females still took the heaviest luggage checked in, but disappointingly only about 0.2 - 0.3 kg more.
People from South America, always seemed to have the heaviest bags, draw your own conclusions there!!
People in first class carried the most baggage, shock horror.
The largest women by departure point were from North America, the men were a reasonably even split but the lightest coming from "Spain, Portugal and Italy" (They were lumped together!) So maybe there is something to the Italian Stallion after all!! The largest men were travelling to "Other Eur. Countries (Including Russia) and the largest women were heading to the "UK, Ireland" and "Scandinavia and Iceland".
Low cost travellers generally weighed less than their full-cost travelling counterparts.
People on Business in Summer weighed 6kg more than their Leisure counterparts and 10kg more in Winter... the Business Traveller always weighed more though
On Average, almost 10kg more.
25% of people carried more than the allocated CARRY-ON baggage allowance, no stats on those who got away with it and those who were pulled up on it though.
Anywho, lots of fun facts for those interested in having a read through the results at the end of the linked survey.