Compressibility factors below one in real gases are a direct consequence of intermolecular attractive forces dominating over repulsive volume effects under specific temperature and pressure conditions, a foundational concept in thermodynamics that requires no citation.
The claim describes a fundamental, textbook thermodynamic property of real gases where attractive forces (represented by the 'a' term in van der Waals-type equations) pull molecules together, making the molar volume smaller than that of an ideal gas and causing the compressibility factor Z to drop below 1. While papers discuss real gas behavior and equations of state, this relationship is a foundational physical principle taught universally in introductory chemistry and physics, qualifying it as common knowledge.