It is my opinion that the % multiplier [as applied to difficulty] needs to be adjusted from where it is now with regard to years in office.
For the 10 years to receive the highest multiplier, ..., generating some of the highest scores possible due to happy citizens, and building value getting multiplied exorbitantly.
I can see how other people may find the difficulty of the game different at different points than myself, but I'm sure most will agree that the bonus % at the shorter duration games is far to high.
I believe that Mjara made some incorrect assumptions. Additionally, several posts wander away from her main point to the issue of rising and/or falling levels of difficulty.
I think that the assumption that the people are happier in the early years and that is the reason that gameplay is easier is unfounded. They are only tolerant. The introductory letter in the manual warns:
Initially, your people are willing to tolerate poor conditions and give you a political 'honeymoon', but if things don't improve quickly, they will start to grow upset with you.
Further, there are four "victory conditions" which produce scores. Those scores are frozen at 10, 20, 30, 40, 50 & 70 years no matter how long the player chooses to play. So the last possible scoring year is 2020. None of the victory conditions apply "happiness" to value of buildings. Only two of them consider citizens.
All of map set-up conditions adjust the difficulty percentage -- length of game is only one of the adjustments. Length is related to the ability to accumulate money and buildings; and also to the time needed to make a real impact on the citizen's happiness. I simply can't accept the assumptions.