Similar to emotional infidelity or sexual infidelity, financial infidelity involves lying about or purposefully withholding information regarding your personal finances from your partner. Financial infidelity can take many forms, including stealth spending (or secretive spending), lying about your income, hiding money from your partner, and opening credit cards without your partner’s knowledge.
Historically, specific adulterous behaviors were universally agreed upon. Today, specific adulterous behaviors vary greatly from relationship to relationship. For example, in modern history, having sex with a person outside of a relationship was considered infidelity. However, today polycules and open relationships mean that having sex with a person outside the relationship is not necessarily considered infidelity. Therefore, the specific behaviors that are considered financial infidelity are defined by you and your partner.
As partners, you are engaging in a mutually agreed upon social contract. Ultimately, breaches to that contract are considered infidelity.