Is a "typical" jam sweeter today than the typical jams of centuries ago? I have a new early morning snack this week - toast and jam, but after putting the (fridge temp) jam on the toast, I put the whole plate of toast and jam in the microwave for 20 seconds to warm the jam. It's delicious, but some of the jams in my fridge are just way too sweet for me this way. That got me thinking - hundreds of years ago, jams were eaten at room temperature. But now they are eaten cold from the fridge as often as not. I wonder if the sugar level has been increased as a result - either by adding more sugars or by reducing the fruit more than before which would also intensify the flavours.

Has anyone compared modern and very old jam recipes to see if this is a real trend? Or the sugar levels in supermarket jam compared to jam recipes in older books?
On the contrary, most of the older recipes that I've seen for jams use more sugar (60g per 100g or higher is common) than is popular in some of the newer products, because sugar has a preservative effect. But most likely pre-refrigeration and pre-iceboxes you simply wouldn't have kept multiple jars open at the same time; you'd use one at a time as quickly as practical, and keep your reserve stock seals unbroken.

Industrially refined pectin has actually reduced the amount of sugar necessary to make a good preserve. A number of companies also now make "all fruit" spreads, but they're controlling the sugar ratio with knowledge from food science, by analyzing how much sugar is in the non-primary fruits like grape and apple juice.

60 grams of fruit per 100g or more are available but by traditional standards are quite a luxury, even though I would agree that such products often taste better; they owe their existence to better food science, however, and a little to refrigeration.

A lot of widely-distributed brands still use sugar ratios that are pretty high, including Welch's and Smucker's, but you may find higher fruit ratios in boutique brands.