I think the biggest things the economists get wrong is that market trend towards an equilibrium value given a sufficient amount of time.
Economists also forget the human factor that is involved in buying/selling instruments. People may buy MSFT because they like the software, people may buy XOM because they see oil prices going up. Some people believe in biotechnology so they buy biotech companies. Investors are not the optimizing automatons that economists think they are.
Additionally, not everyone has the same valuation of returns on a single asset. Not everyone has the same investment horizon. Sometimes people sell securities because they need money. Sometimes people buy securities because have extra money to invest.
Perhaps the biggest problem with the efficient market hypothesis is that there will be no way to beat the market on a return/risk basis because arbitraguers would close the gap.
However, what economists forget is the shear size of the markets. How many billions will it take to close a merger arbitrage, push the spread together on two bond instruments, for mutua funds to adjust their portfolios after a news announcements?
If markets were truely efficient, markets would not exist. Noone would have a compulsion to trade.
Economists also forget the human factor that is involved in buying/selling instruments. People may buy MSFT because they like the software, people may buy XOM because they see oil prices going up. Some people believe in biotechnology so they buy biotech companies. Investors are not the optimizing automatons that economists think they are.
Additionally, not everyone has the same valuation of returns on a single asset. Not everyone has the same investment horizon. Sometimes people sell securities because they need money. Sometimes people buy securities because have extra money to invest.
Perhaps the biggest problem with the efficient market hypothesis is that there will be no way to beat the market on a return/risk basis because arbitraguers would close the gap.
However, what economists forget is the shear size of the markets. How many billions will it take to close a merger arbitrage, push the spread together on two bond instruments, for mutua funds to adjust their portfolios after a news announcements?
If markets were truely efficient, markets would not exist. Noone would have a compulsion to trade.


