Sure there is, we discuss it in economics, finance but yes it changes.
Now regarding long run returns, there are no guarantees. Had money in stocks for college fund, lost half in 08 down turn. Sure it came back but not in time. Friends invested in state run college fund lost everything. Fortunately I also invested in farm land, didn't bet the farm and the farm income paid for the 1st in college. Stocks paid for the 3rd, had to sell stock but not land.