Is it true that forced air distribution systems make the house drier in the winter?
Possibly, but not necessarily. Forced air distributions systems only make the house drier if they are not working properly. If there are large leaks in the return ductwork then dry outside winter air will be drawn into the house through those leaks. This will tend to dry the air in the house. Conversely, hot water systems that are not working properly sometimes add moisture to the house. Steam or hot water systems usually have a pressure relief valve which releases moisture to the room when the pressure in the system gets too high. The main reason for houses being dry in the winter is building envelope leakage. Warm indoor air leaks out through holes in the attic plane, and is replaced by cold outside air leaking in at the floor. This cold air is dry compared to the warm air that was lost. The colder it gets outside, the more air leaks out of those holes.