 As you get older and you don't do anything or you're sedentary, it actually shrinks 1% to 2% a year. I think it believes it starts at 40% and it starts to shrink after that. But your body releases BDNF as you work out, and that helps grow new neurons and grows the hippocampus actually 1% to 2% a year, offsetting the decrease in the hippocampus as you age on.