underdiversification

Noun

uncountable

Insufficient diversification.

Japanese Meaning

十分な分散がなされていない状態 / 分散不足

The investor's portfolio suffered from underdiversification, leaving it vulnerable to sector-specific downturns.

投資家のポートフォリオは分散投資が不十分で、特定のセクターの下落に対して脆弱だった。

Sense（1）

Insufficient diversification.

This is likely because overconfidence is the driving force behind both excessive trading and underdiversification.