This high volatility is one of the reasons why poor countries have no access to the world financial markets. A large number of low income countries are unable to borrow on international markets, either by issuing sovereign debt or by obtaining loans from foreign private banks. Their main source of financing is external borrowing from official concessional sources. Therefore very few market indicators are available to signal the risk of default on this external debt. Interest rates are usually very low and bear no real connection to the risk of non-repayment. This exclusion from financial markets can be the materialization of a very high probability of default for these countries, which increases considerably the borrowing cost for the debtor. For very high probabilities of default, the market is only willing to lend at high values of spreads, which in turn prevent the country to borrow and translate into exclusion from capital markets.