One Copper Trader Hid Losses for a Decade
Sumitomo disclosed enormous losses in 1996 from unauthorized copper trading that had continued for roughly ten years. The duration is the part that should be studied.
The Disclosure
In 1996 Sumitomo Corporation announced losses of roughly 2.6 billion dollars arising from unauthorized copper trading by a single employee, Yasuo Hamanaka, who headed its copper trading operation.
The trading had reportedly continued for about a decade. During that period he was regarded as one of the most influential participants in the global copper market, controlling a share of trading large enough to earn nicknames reflecting his perceived dominance.
How Losses Persisted So Long
The pattern resembles other rogue trading cases with one important difference: the timescale. Most such episodes unravel within months. This one continued for years.
The mechanics involved concealing losses in unauthorized accounts and using falsified documentation to support positions. Because he controlled both trading decisions and significant aspects of the record keeping for his book, the reports flowing upward reflected what he chose to report.
The apparent profitability was the concealment. An operation reporting exceptional returns invites congratulation rather than examination.
The Market Manipulation Element
What distinguishes this case from a simple concealment is that the position was large enough to influence the physical copper market itself.
By accumulating large long positions and controlling substantial physical inventory, the operation could support prices above where supply and demand would otherwise have set them. That elevated price validated the position's value, which supported the reported profits.
The arrangement is self reinforcing while it can be maintained and collapses when it cannot. Once the position had to be reduced, the price fell, which increased the losses on what remained.
Why Governance Failed
Several structural weaknesses recur across these cases. One individual held authority over both trading and elements of settlement for an extended period. He remained in the same role for many years without rotation. And the operation's outsized reported profits reduced rather than increased scrutiny.
The last point deserves emphasis because it is counterintuitive. Risk management attention naturally flows toward operations that are losing money. An operation reporting returns far above what its stated strategy should generate is a signal that either the strategy is not what is described or the numbers are not what they appear.
The Control That Addresses It
The most effective specific control is mandatory consecutive leave. If a trader must be absent for a continuous period and someone else takes over the book, ongoing concealment becomes very difficult to sustain, because the substitute encounters the positions directly.
This requirement is now standard at regulated institutions, and its origin lies in exactly this class of case. It is a cheap control that addresses the specific failure mode of long running concealment.
The Bottom Line
Sumitomo's losses persisted for a decade because the trader controlled his own records and his reported success discouraged questions. Unexplained outperformance is a risk signal, not a reward signal.