My shingles are not that old but they are curling. What causes shingles to cup and curl?

I have seen shingles that curl due to a manufacturers defect but most often the problem is due to an overheated attic space. I have been in attics that are over 100 degrees Fahrenheit. When the attic is poorly ventilated it creates an air trap and acts more like an oven then an attic. As a result the shingles may cup and curl. You might also find shingles that have alligatoring or severe cracks. From my experience I believe the three tabbed shingles are more likely to deform due to the heat described above. Best advice is to add vents in the soffits gables and roof to promote air entry and exit.