a. Low birth rates mean that the share of the population that is working-age will fall over the next few decades. What impact will this have on spending on Medicare and Social Security? The share of GDP spent on these programs will decrease. The share of GDP spent on these programs will increase. Spending on these programs will increase, but the share of GDP spent on them will decrease. Spending on these programs will increase, but the share of GDP spent on them will remain the same.