I'm pretty sure the answer is here, but I don't fully understand it yet,
http://www.investopedia.com/articles...#axzz1wH32Kf9x
"Covered Interest Rate Parity
According to covered interest rate parity, forward exchange rates should incorporate the difference in interest rates between two countries; otherwise, an arbitrage opportunity would exist. In other words, there is no interest rate advantage if an investor borrows in a low-interest rate currency to invest in a currency offering a higher interest rate.
Under the covered interest rate parity condition, the cost of hedging exchange risk negates the higher returns that would accrue from investing in a currency that offers a higher interest rate."
"After one year, the investor receives 105,000 of Currency B, of which 103,000 is used to purchase Currency A under the forward contract and repay the borrowed amount, leaving the investor to pocket the balance - 2,000 of Currency B. This transaction is known as covered interest rate arbitrage.
Market forces ensure that forward exchange rates are based on the interest rate differential between two currencies, otherwise arbitrageurs would step in to take advantage of the opportunity for arbitrage profits"
"The Bottom Line
Interest rate parity is fundamental knowledge for traders of foreign currencies. In order to fully understand the two kinds of interest rate parity, however, the trader must first grasp the basics of forward exchange rates and hedging strategies. Armed with this knowledge, the forex trader will then be able to use interest rate differentials to his or her advantage."