What Is Human Capital?
Photo by Christian Velitchkov on Unsplash
Human capital is the present value of all future labour income — the sum of every salary, bonus, and earned payment an individual will receive over their working life, discounted back to today. For most people in the early stages of their career, human capital is by far their largest asset, dwarfing any financial portfolio they have yet accumulated.
A 25-year-old professional earning £50,000 per year, with 40 years of working life ahead and a modest 3% discount rate, has human capital of approximately £1.2 million. Their financial portfolio might be £20,000. The financial portfolio is less than 2% of their total economic wealth. Decisions about the financial portfolio should reflect the existence of the other 98%.
Human Capital as a Bond
Photo by Jakub Żerdzicki on Unsplash
The key insight is that most people's human capital resembles a bond more than an equity. A stable salaried job delivers regular, contractually determined payments that are relatively insensitive to market conditions. Like a bond's coupon stream, the salary arrives predictably. It does not go to zero in a bear market.
If human capital is bond-like, and the goal is to hold an appropriate overall allocation across total wealth, then a young investor with a large bond-like human capital should hold equity-heavy financial capital to offset it. The total picture — human capital plus financial capital — should reflect the target allocation. Since human capital is already weighted toward bonds, the financial portfolio should tilt toward equities to balance the whole.
As the investor ages and works less, human capital depletes. The bond-like stream of salary income diminishes and eventually disappears at retirement. This is the moment when the financial portfolio must take over as the primary income-generating asset — at precisely the time when it should be positioned more conservatively, with more bonds and income-generating assets that replicate the lost salary stream.
Bodie, Merton and Samuelson (1992)
Photo by National Cancer Institute on Unsplash
Zvi Bodie, Robert Merton, and William Samuelson formalised this intuition in a 1992 paper that gave the life-cycle model its mathematical foundation. A key insight they introduced was the role of labour flexibility. A young investor who suffers large financial losses has a powerful response available: work harder, work longer, delay retirement, save more. This flexibility acts as a form of insurance — it means the effective risk capacity of young investors is higher than it appears from financial capital alone.
As investors age, this flexibility diminishes. A 60-year-old has fewer working years available to recover from a financial catastrophe. The glide from equity-heavy to bond-heavy allocation is not just about time horizon — it reflects the progressive loss of this human capital insurance.
Beyond Age: Individual Differences
Photo by Eugene Zhyvchik on Unsplash
The life-cycle model is richer than the simple "young = equities, old = bonds" rule. The nature of the human capital matters, not just its size. A tenured civil servant has highly stable, bond-like human capital and can safely hold more equities in their financial portfolio. A freelance entrepreneur in a cyclical industry has volatile, equity-like human capital — when markets crash, their business income may also fall. Their financial portfolio should carry less equity risk to compensate.
Similarly, an investor with a defined benefit pension has significant bond-like wealth that is not counted in their financial portfolio — another reason they can accept more equity risk in investable assets. The life-cycle model is a framework for thinking about total wealth, not just the visible investment account.