Or maybe they are both caused by the same underlying cause?
The underlying cause to Psoriasis and gum diseases is use of Antibiotics , in connection with poor diet, poor lifestyle, stress and predisposition to the problem.
That in turns causes damage to the liver, intestines and damage to intestinal microbiota.
Liver damage and intestinal damage and damage to intestinal microbiota in turn affect total ballance of living organisms all over the body, including the mouth, gums, teeth, lips, skin, ears, hair, vagina, feet etc.
Stupid researchers always complicate their conclusions.
All they have to study is relationship between the use of broad spectrum oral Antibiotics , gum diseases and psoriasis, and they would figure it, hopefully one day.