Government Investment

Or it should be expressed in"Government Donation." It is the thing that bothers me and make me rethink how it will contribute the activities of economy better. Keynes mentions that government donation into the country, public facilities building as an example, can make a growth on the national GDP. WHY?? The reason flashed through my head is Modeling Effect, which means that the government shows confident of good investing environments to the investors. But it should be contributed into the "Investment Section." To this, can we say that government investment does no donation to the national GDP? Professor Mao had mentioned about this problem approximately 2 years ago:"If our government made a lot of worthless bridges for our nation, we will still have a growth of economy?"