Let's consider the topic of safe-haven currencies.
A safe-haven currency is one which does well in bad times.
So when the global economy becomes riskier
or something bad happens,
some currencies tend to appreciate
because they are considered safe havens for investors.
The classic example here is the US dollar.
During the American financial crisis of 2008-2009,
a lot of really bad things happened to the United States economy.
Yet, oddly, at these exact same times,
quite often the value of the US dollar would rise on world markets.
This may seem counter-intuitive, but it's reflecting a safe-haven effect.
People figure that if things get really bad,
well, a lot of different assets and asset classes will be in trouble,
and they ask themselves,
"Well, which is actually the safest of those remaining?"
And the US dollar, actually, in the minds of investors,
does pretty well on that score.
Another safe-haven currency, very frequently, is the Japanese yen.
And another safe-haven currency is the Swiss franc.
Like Japan, Switzerland has a reputation for being a very sound and stable place.
It's certainly perceived as such by investors.
So during bad or risky times,
there's often an inflow of money into Switzerland
and an appreciation of the Swiss franc.
What are the factors behind what might be a safe-haven currency?
Well, there's some disagreement here,
but overall some of those factors would include
the long term stability of purchasing power in that currency,
the general geopolitical outlook for that country,
its monetary and fiscal policies and how good those are,
the size and safety of the country;
that's one factor favoring the United States;
whether that country has a significant role in global trade
including the use of that currency as a reserve currency;
that would probably be robust even in bad times.
Quite important is the liquidity of markets for that currency
and, also, the net asset position of the country;
simply, how much wealth has that country accumulated
relative to how much it owes other parties.
There are some other potential safe-haven current,
sometimes, but not always cited.
Those would include the euro, sometimes, the Korean won,
or even the Norwegian crown.
If you go back to the previous list,
you'll find that these are not nearly as safe haven, at least as of 2013,
as the US dollar, the Japanese yen, and the Swiss franc.
It might sound like a good thing to be holding a safe-haven currency,
well, maybe it is because, in bad times, it gives you more protection;
that's a kind of insurance.
But, keep in mind, there's always trade-offs in economic activity.
So a currency which is giving you more insurance protection upfront
is probably paying you, all other things equal, lower returns.
So, just keep this in mind,
there is a trade-off between risk and return
when considering the costs and benefits of holding safe-haven currencies.
For more on this topic, the best place to start
is simply to put "safe-haven currency" into Google
and also into scholar.google.com.