Brad DeLong cuts through a century of academic fog to reveal that Karl Marx was not oblivious to the mathematical contradictions in his own labor theory of value; rather, he understood them perfectly and chose to treat them as secondary corrections. This piece is notable because it dismantles the common critique that Marx simply failed to do the math, replacing it with a more nuanced historical reality: the "transformation problem" was fully articulated by Marx in 1862, long before critics claimed it was an oversight. For busy readers trying to grasp why economic models often fail to predict real-world crises, DeLong offers a masterclass on the difference between a useful simplification and a broken assumption.
The Myth of Mathematical Incompetence
DeLong immediately challenges the narrative that Marx was intellectually dishonest or simply confused about how profits are equalized across industries. He writes, "This contradiction was not because he was unaware of the 'transformation problem': he was very aware of it, and in fact slagged Ricardo for not seeing it sufficiently clearly." This reframing is crucial. It shifts the debate from whether Marx could do algebra to why he prioritized the origin of profit over its distribution.
The author argues that for Marx, the labor theory of value was a "sharp knife" designed to dissect where profit came from—surplus value extracted from workers—not necessarily to predict every market price with precision. DeLong notes, "Karl Marx thought that because profit was 'really' surplus value, the way to analyze the category of profit was (i) to look at its origins in labor exploitation, and then (ii) track its transformation into profit." This distinction is vital for understanding why Marx believed average market prices would cluster around labor values, even if they weren't identical. Critics might argue that ignoring the divergence between value and price makes the theory empirically weak, but DeLong suggests this was a deliberate theoretical choice to highlight exploitation rather than obscure it with complex pricing mechanics.
All economic models are, as Bob Solow wrote back in 1956: 'All theory depends on assumptions which are not quite true. That is what makes it theory.'
DeLong leans heavily on this insight from Nobel laureate Robert Solow to defend the validity of imperfect models. The core of his argument is that a model must be wrong enough to be simple, but not so wrong as to be useless. He writes, "Economic theory is a very strange beast—it has to be wrong, or else your map is the size of the territory, and hence is useless; but it cannot be too wrong, or else it is useless, but in a different way." This is a powerful defense against critics who demand mathematical perfection from 19th-century texts. It forces the reader to ask: what is the model actually trying to explain? If the goal is to understand power dynamics between capital and labor, perhaps price deviations are noise.
The Failure of Modern Formalism
The commentary takes a sharper turn when DeLong addresses modern economists who claim that if Marx had just written down formal equations, his errors would have been obvious. He mocks this sentiment by pointing out the failures of contemporary giants like Robert Lucas and Edward Prescott. "Writing equations does not save you," DeLong asserts, noting that these theorists often rely on assumptions that are demonstrably false regarding how humans actually perceive prices.
He dismantles the "island model" used by Lucas, which assumes workers know selling prices but are confused about buying prices. DeLong counters with a stark observation from real life: "In my experience, however, it is very much the reverse I very much know very much the prices at which I am buying things." This personal anecdote serves as a potent critique of abstract modeling that ignores human behavior. He argues that if anything, nominal shocks should reduce employment because workers are clearer on their costs than their future earnings, yet Lucas's model predicts the opposite.
Furthermore, DeLong attacks the methodology of using "unmoved prime movers" to explain economic fluctuations. Regarding Prescott, he notes, "he made it very clear that it was illegitimate to inquire into the technological shocks were: one simply had to take the residuals from a production function and plug them in." This is a damning indictment of treating unexplained data as an exogenous force rather than investigating the actual mechanisms at play. The reference here connects back to the historical context of the transformation problem; just as Marx saw through Ricardo's conflation of value and price, DeLong sees through modern economists' conflation of mathematical elegance with economic reality.
Historical Depth and the 1862 Letter
To prove his point about Marx's foresight, DeLong dives into a specific historical artifact: an August 1862 letter from Marx to Friedrich Engels. In this correspondence, written five years before Capital was published, Marx explicitly details how competition equalizes profit rates despite different capital compositions. He writes, "Ricardo confuses value and [average] cost price... [This] identification of values of commodities and [average] cost prices of commodities is totally wrong."
DeLong uses this primary source to show that the "transformation problem" was not a later discovery or an accidental omission. Marx had already worked through the mechanics where different ratios of constant capital (machinery, raw materials) and variable capital (wages) lead to different rates of surplus value, which competition then averages out. He illustrates this with a detailed breakdown of four hypothetical industries, showing how a uniform rate of profit emerges from unequal exploitation rates in specific sectors. This historical grounding adds significant weight to his argument that Marx was a rigorous thinker who understood the limitations of his own maps.
The art of successful theorizing is to make the inevitable simplifying assumptions in such a way that the final results are not very sensitive... When the results flow specifically from a special crucial assumption, then if the assumption is dubious, the results are suspect.
Bottom Line
Brad DeLong's analysis succeeds by refusing to let Marx off the hook for being "right" while simultaneously refusing to let modern critics off the hook for being lazy. The strongest part of this argument is its insistence that the value of a theory lies in its ability to reveal hidden structures—like surplus value extraction—even if it fails to predict every market price. However, the piece's biggest vulnerability is its reliance on the reader accepting that Marx's "confusion" about his own shifting thoughts is actually a feature, not a bug, of deep theoretical work. Ultimately, DeLong reminds us that in an era of increasingly opaque economic models, we should be wary of anyone who claims their equations perfectly capture reality.