One of the factors in the financial crisis of 2007-2009 that is mentioned too infrequently is the role of banking capital sufficiency standards and exactly how they were written. Folks have said that capital requirements were somehow deregulated or reduced. But in fact the intention had been to tighten them with the Basil II standards and US equivalents. The problem was not some notional deregulation, but in exactly how the regulation was written.
In effect, capital sufficiency standards declared that mortgage-backed securities and government bonds were "risk-free" in the sense that they were counted 100% of their book value in assessing capital sufficiency. Most other sorts of financial instruments and assets had to be discounted in making these calculations. This created a land rush by banks for mortgage-backed securities, since they tended to have better returns than government bonds and still counted as 100% safe.
Without the regulation, one might imagine banks to have a risk-reward tradeoff in a portfolio of more and less risky assets. But the capital standards created a new decision rule: find the highest returning assets that could still count for 100%. They also helped create what in biology we might call a mono-culture. One might expect banks to have varied investment choices and favorites, such that a problem in one class of asset would affect some but not all banks. Regulations helped create a mono-culture where all banks had essentially the same portfolio stuffed with the same one or two types of assets. When just one class of asset sank, the whole industry went into the tank,
Well, we found out that mortgage-backed securities were not in fact risk-free, and many banks and other financial institutions found they had a huge hole blown in their capital. So, not surprisingly, banks then rushed into government bonds as the last "risk-free" investment that counted 100% towards their capital sufficiency. But again the standard was flawed, since every government bond, whether from Crete or the US, were considered risk-free. So banks rushed into bonds of some of the more marginal countries, again since these paid a higher return than the bigger country bonds. And yet again we got a disaster, as Greek bonds imploded and the value of many other countries' bonds (Spain, Portugal, Italy) were questioned.
So now banking regulators may finally be coming to the conclusion that a) there is no such thing as a risk free asset and b) it is impossible to give a blanket risk grade to an entire class of assets. Regulators are pushing to discount at least some government securities in capital calculations.
This will be a most interesting discussion, and I doubt that these rules will ever pass. Why? Because the governments involved have a conflict of interest here. No government is going to quietly accept a designation that its bonds are risky while its neighbor's are healthy. In addition, many governments (Spain is a good example) absolutely rely on their country's banks as the main buyer of their bonds. Without Spanish bank buying, the Spanish government would be in a world of hurt placing its debt. There is no way it can countenance rules that might in any way shift bank asset purchases away from its government bonds.