
In the attached chart you will see how the dollar and the stock market are tied at the hip. Except for the late 90's (an anomaly in so many ways), the dollar and the stock market lead each other inversely.
Even more than the stock market, the dollar is tied to the debt market (which also leads stocks). The reason is interest rate parity, or lack thereof.
When a country has a high interest rate, other people in countries with lesser rates will park money in the higher rate country. This raises the price of that currency (demand up) and thus lowers the rate of their home currency (demand down). When the lesser rate country returns the money back home, its fx rate is now weaker and it wins on both accounts (higher int. rate and gain in currency). If you are familiar with the carry trade that was so popular in the 2000s, then that is similar. This market "arbitrage" also goes against the interest rate parity theory that states that countries with higher interest rates must have lower future currency rates to offset that gain.
People flock to currencies with high interest rates and that is why the dollar hit its peak in the early 80s (as interest rates peaked) and is now at an all time low (as interest rates are at an all time low). During the early 80's stocks rallied hard (the 80's bull was actually bigger than the 90's as the dollar fell 30-40%). The chart above explains why, and I will explain in an example as well.
Most items are sold "in US Dollar terms". This means that the price (denominator) is the dollar. The toy you buy your kid for xmas can be translated as toys per dollars to come up with what it will cost you to purchase it. If you could trade a blanket for that toy it would be denominated as toys per blankets to come up with its value. The same is true for the stock market.
If the stock market's earnings in 1985 were $10/year and people were willing to pay 10x for those earnings then it is priced at $100s per 1 S&P500. This can be translated as $100/1 USD (with the "1" meaning 1 US Dollar which happens to be trading at an all time high of $1.65). Assuming earnings and multiples stay the same, if the $USD then weakens to $1.00 (which it did), then the real value of your stocks just jumped 39% (translating $1.65 down to $1.00), and in order to remain whole and offset that change, the S&P500 would go to $139, just by updating the reporting unit!
An easier way to think about this is thru inflation. When the value of the dollar falls, one way to conceptualize this is to think about there being more of them out there representing the same one thing, aka now worth less per dollar. This is basically inflation. If the denominator falls, the numerator is now instantly worth more (going from 10 toys/2 blankets=5 unit cost down to 10 toys for 1 blanket=10 unit cost).
The practical way to think about this is to replace the $USD with something more tangible like milk, or a house, or a gold bar! Why measure wealth in the $USD anyways? The $USD is only as good as its purchasing power, so why not replace it with something of more consistent purchasing power? If you replace the denominator with the more tangible asset like the price of gold (which many people do), you will see that stocks have actually lost a ton of "real" value since the 2000 top. At that top stocks were worth about 5.6x an ounce of gold. Today they are worth less than 1 ounce. Chart attached below. So basically you used to be able to sell your stocks for 5.6 ounces of gold but now it will only buy less than 1 ounce of gold. Assuming an ounce of gold today has the same utility as an ounce of gold in 2000, stocks have fallen in value significantly. You can do the same with oil, commodities, water, shelter, or any other valuable, measurable necessity. Most of them will show a decline in the value of stocks over the last decade. Chart of the S&P priced in Gold, below.

The key takeaway is that stock prices are only one component of their worth (numerator). Don't forget the denominator piece (the US Dollar) as that is just as important in establishing a stock's true value. Thus, if you can get a sense of where the dollar is heading then you are 50% of the way to finding out what your stock's true value is. Another takeaway is that as long as the dollar is falling, then items priced in dollars should be going up in price, stocks included...and viceversa.
Supple this to my previous blog post on the bottoming US Dollar and we might be in for a beating in the stock market depending on the size of the move. There haven't been that many periods since the early 80s where the dollar has risen in value (except the bubble 90's where everything went up - stocks, bonds, dollar), but there are many when the dollar has fallen hard...and most of those times saw stocks rally. We got a glimpse of what a dollar rally can do to stocks in the early 2000s as well as 2008. We also know that the dollar rallied with interest rates in the 70's and early 80's when stocks were flat to down. In 2005 the dollar rallied and stocks stayed relatively flat (compared to surrounding years when the dollar fell hard and stocks rallied hard). The math also works to support this thesis. Right now (previous blog post), the dollar is looking ripe for a rally. It will be interesting to see what effect that has on stocks.
Pay close attention to that dollar!