The Neuroscience of the Sirens' Song

Homer was clearly on to something. Modern neuroscience suggests that physiological mechanisms in our brains help explain many of the behaviors documented by behavioral finance.

One intriguing finding is that our brains don’t process anticipated gains and losses symmetrically. When we imagine future rewards, a subcortical region known as the nucleus accumbens becomes active. This region is rich in dopamine and is associated with reward, motivation, and reinforcement. The anticipation of gains activates this system, generating a physiological signal that can amplify optimism before any gains are actually realized. This suggests that the anticipation of future rewards can become reinforcing in its own right – a dynamic that may help explain why speculative booms often become emotionally intoxicating. Notably, this is the same reward circuitry that has been implicated in gambling addiction.

Anticipated losses, however, tend to be processed through completely different neural systems, including regions such as the amygdala and insula, which are more closely associated with threat detection, aversion, fear, and bodily discomfort. In this sense, our brains simply don’t respond to the prospect of losses the same way we do to gains.

That asymmetry may help explain a familiar feature of financial markets: investors often worry the least about risk after markets have risen substantially and become much more risk averse after markets have already fallen. During prolonged advances, rising prices do more than increase wealth; they reinforce confidence and the expectation of future returns. As investors imagine future gains, dopamine-mediated reward systems are repeatedly activated. Risk doesn’t disappear, but it becomes much easier to ignore. The psychological weight assigned to potential rewards begins to exceed the weight assigned to potential risks. Existing risks can remain right in front of us, but they become much easier to discount, rationalize, or overlook.

When markets finally do reverse, the process often flips in the opposite direction. As anticipated gains fade, so too does the neurological reinforcement associated with reward circuitry. Risks that were present all along suddenly come into sharper focus. Investors often experience this as a new realization, but the underlying risks usually haven’t changed anywhere near as much as perceptions have. The rocks never moved. The Sirens' song simply made them easier to overlook.

This observation may initially appear inconsistent with one of the most influential findings in behavioral finance. Daniel Kahneman and Amos Tversky famously demonstrated that people tend to experience losses more intensely than equivalent gains. Losing one hundred dollars generally hurts more than gaining one hundred dollars feels good. This principle, known as loss aversion, became a cornerstone of Prospect Theory and helped explain a wide range of investor behavior.

Yet the two ideas are not at all contradictory. They’re simply describing different stages of the decision-making process.

The neuroscience of reward anticipation helps explain why investors become captivated by the potential for gains before outcomes are known. Prospect Theory helps explain what happens after gains and losses begin to feel real. One helps explain why investors steer toward the Sirens. The other helps explain what happens when the rocks suddenly come into view.

Prospect Theory contains another important insight that helps explain investor behavior during market declines. The pain associated with realizing a loss can become so psychologically powerful that many investors prefer a gamble offering the possibility of recovery over accepting a certain loss. Decision-making shifts away from discipline and toward recovery. Rather than accept a loss, investors frequently choose to take on additional risk in the hope of getting back to even. This helps explain why declining markets often provoke not caution but escalation – averaging down, increasing exposure, or doubling down on positions that are already under water. At some point, however, as losses deepen and the prospect of recovery begins to feel increasingly hopeless, this risk-seeking behavior can give way abruptly to capitulation, with investors shifting almost instantly from trying to get back to even to simply preserving what they have left.

During speculative advances, anticipated gains activate reward systems that focus attention on opportunity and reinforce optimism. As prices rise, investors increasingly imagine future rewards, and those imagined rewards themselves become psychologically persuasive. Risks remain present, but they carry less weight. The Sirens' song grows louder.

When markets reverse, the same investors often undergo a dramatic shift in perception. The anticipated rewards that once dominated attention recede while potential losses become harder to ignore. Loss aversion begins to exert greater influence. Investors become more sensitive to downside risk, more focused on preservation, and less willing to take on uncertainty. In many cases, they become most risk averse only after prices have already fallen substantially.

This helps explain one of the enduring paradoxes of financial markets. Investors often become least concerned about risk when risk is greatest and most concerned about risk after the bulk of the damage has already been done. The underlying fundamentals generally change far less than sentiment, attention, and perception.

Seen through this lens, speculative cycles are not simply stories about greed and fear. They are stories about how humans process opportunity and danger. Speculative cycles reflect the interaction of distinct neurological and psychological mechanisms. Reward-seeking systems encourage investors to focus on opportunity during booms, while loss aversion and threat-detection systems become increasingly dominant during busts.

This is also where value investing and contrarianism enter the discussion. The discipline required to buy when markets are plummeting, or to resist what others have been chasing, runs directly against the emotional currents created by markets themselves. Warren Buffett captured the idea perfectly: "Be fearful when others are greedy and be greedy when others are fearful." The difficulty is that this advice sounds simple only in hindsight. In real time, greed often arrives wrapped in confidence, consensus, and recent gains, while fear arrives wrapped in falling prices, uncertainty, and the sudden recognition of risks that were present all along. Successful value investing is less about superior intelligence than about resisting the powerful neurological and emotional forces that drive the crowd – and that challenge is much less analytical than it is emotional.

Homer understood the practical implication thousands of years before neuroscience existed. Odysseus couldn’t assume he would remain rational once the Sirens started singing. He knew that temptation itself would impair his judgment. His solution wasn’t intelligence, courage, or willpower. It was preparation. He bound himself to the mast before the music began.

Investors have always faced similar challenges. The greatest dangers in markets rarely arise because risks are invisible. More often, they arise because compelling narratives make those risks easier to ignore. The rocks are always there. The challenge is remembering that fact when the Sirens are singing loudest.

The Song Remains the Same

When Sirens Sing: How the Market Cycle Seduces Investors

Let’s face it, there are stretches when markets can make caution feel irrational. Common sense falls by the wayside and, with each new market advance, restraint comes to be viewed less as discipline than missed opportunity. “As long as the music is playing,” former Citigroup CEO Chuck Prince famously said near the peak of the pre-2008 credit boom, “you’ve got to get up and dance.”

The dancer in question, Chuck Prince, resigned just months later in 2007 as the unfolding subprime crisis revealed Citigroup to be far more fragile than markets believed. A year later, the firm required tens of billions in federal support to survive. Prince’s words endured because they captured something deeper than greed.

During speculative cycles, the pressure to participate becomes psychologically overwhelming – even to a top banker who, one would think, knows better – precisely because parts of the story are true. Rising stock prices create social confirmation. Consensus itself becomes emotionally persuasive. The longer trends persist, the more investors mistake momentum for inevitability and price appreciation as evidence that their underlying assumptions are correct.

Each Siren’s song has fresh lyrics, but they all conform to a pattern Homer described in The Odyssey, an epic in Greek literature written more than 2,500 years ago.

In Homer’s Odyssey, the Sirens were not predators in the ordinary sense. They did not attack ships or overpower sailors by force. Instead, they waited patiently on a rocky island surrounded by the wreckage of those who came before and sang seductive songs. Ancient descriptions vary, but the essential idea remains the same: the Sirens overwhelmed judgment through persuasion. They promised knowledge, revelation, insight. They sang directly to what each sailor most wanted to believe about himself, convincing him that he was sophisticated enough to approach safely and exceptional enough to understand what others could not.

That’s what made them deadly.

The sailors steered themselves directly into the rocks, rendered unable to exercise judgment. Odysseus understood this before he ever approached. He recognized that the real danger wasn’t ignorance, but exposure. Once the music started, he knew he would no longer be able to trust his own judgment. So he prepared in advance, establishing his own precommitments: wax in his crew’s ears, rope lashing himself to the mast, strict instructions that no matter how violently he begged to be released, they must ignore him.

Every frothy market has its own Sirens.

Today they sing about artificial intelligence, momentum, technological inevitability, and the promise that innovation will overcome every structural constraint confronting the global economy. Their song is seductive because the underlying story contains elements of truth. AI is transformational. It may eventually raise productivity, reshape – even disrupt – industries, and create extraordinary long-term economic value for society.

That said, history reveals that the most dangerous market manias are never built on fiction alone. Every prior technological step-change in modern history has left behind its own coastline of bleached bones. Railroads transformed America. Electrification transformed industry. The internet restructured modern civilization. Yet each also produced waves of overinvestment, speculation, leverage, and enormous losses for investors. The lesson: technology usually succeeds. Most investors don’t.

The pattern is not theoretical. Cisco was one of the defining companies of the internet era, and its business ultimately validated much of the optimism embedded within it. Yet investors who purchased the stock at the peak in 2000 waited more than twenty-five years to get back to even in nominal terms – and they’re still waiting if you take inflation into consideration. Likewise, Amazon went on to become one of the most dominant companies in history but still declined more than 90% during the unwind of the same cycle before compounding extraordinary returns from a much lower base. Others, like Enron, were not mispriced versions of real success but outright illusions where narrative, complexity, and momentum completely obscured the underlying lack of substance.

Different outcomes, but a common thread: in episodes where narrative and momentum dominate market pricing, technological progress, intrinsic value, and investor returns can diverge dramatically. These distinctions matter now because markets are already well past being merely optimistic.

The preceding era was defined by secular disinflation, globalization, cheap energy, favorable demographics, falling interest rates, declining term premiums, and a steadily declining cost of capital.

In the aftermath of the Global Financial Crisis, those forces were amplified by zero-interest-rate policies, quantitative easing, abundant liquidity, and repeated central-bank intervention. Together, they created an ideal environment for long-duration growth assets, venture capital, leveraged finance, passive concentration, and buy-the-dip psychology to flourish.

But the regime that produced those conditions no longer exists.

Since the pandemic, inflation has structurally shifted upward. Long-term interest rates broke out of a forty-year downtrend five years ago. Fiscal discipline has steadily deteriorated across much of the developed world. Geopolitical fragmentation has also intensified. Supply shocks are recurring faster than previous dislocations can be fully rectified. The cost of capital is rising, and the tide that once swept everything in a froth of cheap money has long since turned. Yet current market pricing remains anchored to assumptions formed under a very different set of economic conditions.

The great vulnerability today is not merely that stocks are expensive. It is that large parts of the financial system remain priced for an interest rate environment that no longer appears consistent with the more inflationary world around us today. Asset prices, private market marks, refinancing assumptions, and investor behavior all remain dependent on the belief that inflation will fade back toward its old range, that rates will eventually revert downward, that liquidity will remain abundant, and that technological progress will overwhelm all constraints.

AI enthusiasm increasingly appears to be sustaining assumptions inherited from that earlier environment. That is today’s Sirens’ song. AI seems to offer investors a seductive answer to every structural problem. Productivity will offset inflation. Automation will solve labor shortages. Growth will outrun deficits. Scale will justify valuation. Technology will protect margins. Innovation will render old rules obsolete.

Some of that may ultimately prove true. But in the meantime, investors appear to be pricing a collection of plausible outcomes as though they were inevitable.

When bubbles form around transformative technologies, they rarely emerge from pure fiction. As OpenAI cofounder Sam Altman himself observed, speculative manias often begin with “a kernel of truth” powerful enough to justify genuine optimism before eventually encouraging extraordinary excess.

AI is anything but asset-light. Already, the buildout represents one of the largest capital expenditure cycles in modern American history. Datacenters require enormous amounts of electricity as well as vast quantities of land, water, chips, cooling systems, transmission infrastructure, financing, and time. They compete for capital in a world already strained by deficits, rearmament, reshoring, energy transition, and demographic pressure. Our digital future is increasingly constrained by physical bottlenecks.

Large infrastructure projects have a long history of requiring more time and money than initially projected. Permitting delays, labor shortages, rising input costs, financing constraints, and political opposition have repeatedly challenged the economics of major buildouts. History offers little precedent to assume today’s AI buildout will prove uniquely immune to these realities.

Altman himself has openly acknowledged both the scale and speculative nature of what is unfolding. In discussing AI infrastructure financing, he suggested that the world may need “a new kind of financial instrument” to fund the enormous “compute” buildout ahead. Elsewhere, when asked whether investors had become overexcited about AI, he compared the current environment to the dot-com bubble and observed that “somebody is going to lose a phenomenal amount of money.” The warning was striking precisely because it echoed the history of prior transformative technological booms: enormous societal benefit often accompanied by substantial capital destruction for investors.

Consider the Stratos project underway in Box Elder County, Utah. Stratos is projected to occupy a 40,000-acre campus – more than twice the size of Manhattan – while operating an integrated compute-and-power system utilizing roughly nine gigawatts of electricity. For perspective, that’s roughly 4.5 times the output of Diablo Canyon, California’s last operational nuclear plant, which generates about 9% of the state’s electricity. Nor is Stratos the largest such project currently under development. It is merely one visible example of a global infrastructure race whose scale would have seemed unimaginable only a few years ago.

Today’s AI boom looks much less like traditional software cycles than it does the railroad, electrification, and fiber-optic buildouts that transformed earlier generations. This discussion isn’t merely macro. There are important micro ramifications as well.

Investors should recognize that many of the Magnificent Seven have spent years enjoying the economic benefits associated with dominant or near-monopoly positions in largely separate markets. Search, social media, enterprise software, cloud computing, e-commerce, premium consumer hardware, and advanced semiconductors each generated extraordinary returns with relatively limited direct competition from one another. Increasingly, however, many of these same firms are now converging on the same AI opportunity, deploying enormous amounts of capital in pursuit of what must ultimately be a contested prize. As this trend continues, investors may find it increasingly difficult to justify extrapolating the economics of yesterday's monopolies into a future that looks considerably more competitive.

This convergence is not incidental. These kinds of generational capital expenditure booms consume enormous amounts of both capital and physical resources and are inherently inflationary. They invite overcapacity, require financing, and are vulnerable to rising interest rates. Buildouts like we’re seeing now tend to end up rewarding society far more than the investors who finance them in aggregate – especially those who show up late to the dance.

Historically, these are the kinds of conditions in which momentum has proven especially dangerous.

Since the pandemic trough, and especially following the public release of ChatGPT, markets have increasingly become momentum-driven. Leadership has narrowed aggressively into a concentrated group of perceived AI beneficiaries, with the Magnificent Seven now accounting for roughly a third of the S&P 500's total market capitalization. If Alphabet, Amazon, Meta, and Tesla were classified as technology companies, the broader technology sector would represent more than half of the index's market value.

Passive capitalization weighting has mechanically directed larger flows toward the same winners as their market capitalizations expanded. Relative performance pressures have encouraged active managers to chase the same leadership rather than risk underperforming increasingly concentrated benchmarks.

These dynamics may be more powerful today than during previous speculative episodes. Passive mandates now represent a far larger share of equity ownership than they did during the late-1990s technology bubble, increasing the importance of benchmark-driven flows and concentration effects.

This is how we arrive at environments where a handful of companies increasingly dominate benchmarks, passive flows reinforce concentration, and owning the same leadership names becomes increasingly difficult for many investors to ignore. It is classic herding.

Retail speculation has further amplified these dynamics over recent years. Options activity, thematic trading, and social reinforcement loops have accelerated the tendency for rising prices themselves to become interpreted as confirmation of the underlying bullish narrative. In many cases, valuations at the individual company level have expanded far more rapidly than their capacity to generate cash flow, suggesting that narrative reinforcement, behavioral extrapolation, and momentum themselves have become primary drivers of market pricing.

Markets are especially vulnerable to these dynamics because many of the biases governing human decision making become most powerful after prolonged advances. Investors naturally extrapolate recent experience into the future. Consensus becomes psychologically reassuring. Career risk discourages deviation from prevailing narratives while repeated reinforcement gradually erodes skepticism. Over time, investors do not merely begin expecting higher prices; they begin treating the continuation of the trend itself as evidence that their underlying assumptions must be correct.

Momentum works when price trends become self-reinforcing. Rising prices attract attention. Attention attracts investment flows. New investment reinforces the winners. The winners dominate market cap-weighted indexes. Passive flows send even more money their way. Backward-looking performance is perceived as evidence. Evidence then becomes narrative. Narrative becomes consensus.

A related phenomenon occurs when market prices begin influencing the behavior of investors, companies, lenders, and consumers, temporarily shaping the very fundamentals they are supposed to measure. Under certain conditions, prices become more than outputs. They become inputs into the feedback loop. Recent private funding rounds illustrate the point. Rising private valuations at Anthropic generated mark-to-market gains for both Alphabet and Amazon through their minority ownership stakes, significantly boosting reported earnings despite no corresponding change in operating performance and no realization of cash flows. In situations like these, market prices can begin influencing the very fundamentals investors are using to evaluate them.

In influential research on momentum investing, behavioral finance scholars Mark Grinblatt and Tobias Moskowitz found that investors often adapt slowly to major structural changes in the economic environment, allowing mispricing and market trends to persist longer than traditional financial theory would predict. Investors tend to underreact to regime shifts, especially when new realities conflict with entrenched narratives and existing consensus. The persistence of post-GFC assumptions in today’s market may be an example of that dynamic.

These pressures don’t just operate at the individual psychology level. They become embedded institutionally. Professional managers are rarely rewarded for diverging too early from consensus, but they are often punished for underperforming it. Over time, momentum stops being merely a behavioral phenomenon and becomes endogenous to the system itself.

Eventually, investors stop asking, “What is this worth and how is it going to compound from these levels?” and start asking, “How can I afford not to own it?”

That transition is psychological, not analytical.

Grinblatt and Moskowitz also documented how the very forces driving momentum can sow the seeds of reversal when expectations outrun economic reality. Technological revolutions often validate the underlying innovation while simultaneously destroying capital for investors who financed the boom at inflated prices.

Stanley Druckenmiller once described selling technology stocks near the peak of the dot-com bubble because valuations had become absurd, only to watch them continue rising. He understood the danger. He knew better. Yet he eventually bought them back near the peak anyway. “I just had to play,” he later admitted. “I couldn’t help myself.” Decades earlier, Yale economist Irving Fisher described the stock market as having reached a “permanently high plateau” just days before the end of the great momentum-driven rally that preceded the stock market crash of 1929.

These are just a few examples of the Sirens’ song at work.

They don’t make intelligent people stupid, but they can make discipline feel irrational.

The core assumption markets refuse to abandon is that the post-2008 framework remains the relevant one. It’s not. As I wrote about last quarter, the Five Ds – deficits, demographics, deglobalization, defense spending, and datacenter capital expenditures – are already firmly entrenched and placing upward secular pressure on inflation and longer-term interest rates. Massive fiscal irresponsibility amplifies all of them. Recurring supply shocks are not the cause of the new economic order; they merely act as catalysts and accelerators within it.

That distinction is important – temporary shocks fade; structural forces don’t.

This leaves markets in a precarious position. They remain priced for a world of lower rates, expanded multiples, and abundant liquidity while the real world is increasingly defined by rising capital costs, increasing fiscal strain, geopolitical fragmentation, energy constraints, and structurally persistent inflationary pressure.

These mismatches can coexist for some time. Markets have always shown the ability to remain expensive longer than skeptics expect, especially when powerful technological narratives become intertwined with momentum and institutional reinforcement. AI enthusiasm likely still has plenty of runway left. Capital continues to flow toward perceived winners, rising prices continue to validate consensus, and strong performance continues attracting additional inflows. Momentum tends to persist right up to the point that it doesn’t.

Warren Buffett once quoted the late Barton Biggs as saying, "A bull market is like sex. It feels best just before it ends." Colorful as the observation may be, it captures something important about speculative cycles. They’re often most seductive just before reality begins to set in.

Historically, this is how momentum-driven markets tend to end. They rarely collapse because investors suddenly lose faith in any underlying technology. More often, financing conditions tighten, expectations outrun economic reality, or leadership becomes so concentrated that even modest disappointments begin forcing investors to reassess previous assumptions. Some cycles unwind violently as reflexive flows suddenly reverse direction. Others simply spend years going nowhere while fundamentals slowly catch up to valuations. Occasionally, enthusiasm diffuses more gradually across the market. But regardless of the path, the same momentum dynamics that once amplified the advance eventually stop functioning as an accelerant.

The bottom line is that the longer faulty assumptions remain embedded in pricing, and the further reality diverges from perception, the more fragile the financial system becomes.

Odysseus survived the Sirens not because he was immune to their songs, but because he understood he would not be immune when the time came. He did not rely on willpower. He had himself bound to the mast before he heard the music.

That is what investment discipline is supposed to do.

Quality investing isn’t a short-term trading scheme. Valuation discipline doesn’t imply timidity. Strong balance sheets, durable cash flows, reasonable prices, and margins of safety are all important precommitments designed to protect investors precisely during the moments when judgment becomes most difficult. The closing lines of the Sirens’ song are always the most enchanting.

They’re in full chorus now.

The Sirens’ song is seductive precisely because it is rooted in something real. But never forget: the rocks are just as dangerous whether the songs ring true or not.