MarketPath
The market engine. MarketPath generates the futures your plan is tested against: monthly paths for stocks, bonds, inflation, and interest rates, simulated as one linked economy that starts from the market you face today.
Why not just use historical averages?
No one retires into the average market. Two retirements with the same average return end in different places when the good and bad years arrive in a different order, and a hard first decade with steady withdrawals can sink a plan the average called safe.
History is also a single sample: the United States in its best century, measured after we know it survived. Replaying it assumes the same luck arrives twice. MarketPath uses history as evidence about how markets behave, and it can produce decades better or worse than any on record.
It starts from today
Every simulation opens at the market you face: current equity valuations, interest rates, and recent inflation. When prices are high relative to earnings, expected long-run returns are lower and drawdowns cut deeper, and the simulated futures reflect that.
Below, the same plan and the same 4% withdrawal rule, started into two different markets. Each fan is the spread of portfolio balances across two thousand retirements simulated by MarketPath: the darker band holds the middle half, the solid line is the median, and the dashed line is what actually happened.
One linked economy
A retirement plan lives in a whole economy. MarketPath simulates US and international stocks, nominal bonds, inflation-protected TIPS, cash, inflation, and interest rates as one connected system: inflation shocks push bond yields up and bond prices down, valuations respond to rates, and recoveries follow busts on historical timing.
The linkage produces the hard scenarios. The 1970s pattern, where stocks and bonds fall together while prices rise, emerges from the same machinery as the ordinary years. So does its opposite, the 2008 pattern, where stocks crash, inflation evaporates, and Treasuries rally as the refuge.
The rare decades matter most
Most of the risk in a retirement sits in rare, extended episodes: a deep depression, a decade of stagflation, a bubble that overshoots and pops. MarketPath generates these at the frequency and depth the record and the research literature support, so a plan's score includes them alongside the long ordinary stretches between them.
How we keep it honest
We backtest the engine across a century of starting points. Given only what was knowable in 1975, or 1990, or 2005, the range it projects has to contain what actually happened, at the advertised rates: the middle 80% of the fan should catch reality about 80% of the time. We also run the classic safe-withdrawal studies through it, from Bengen's 4% work to current forward-looking estimates. When we adopt a study's assumptions, the engine recovers its answer. Where our numbers differ, the difference comes from assumptions we chose on purpose: inflation is simulated rather than held at a constant average, and projections start from today's valuations rather than the historical mean.