MOSCOW — Elena Y. Kotova’s one-room apartment on the outskirts of Moscow had started to feel cramped. Her son was almost 4 years old. She and her husband had solid jobs, she as an economist and he as a manager of a construction company. It was time to buy a new home.

“We just need the space,” she said. They found a two-bedroom apartment and secured a mortgage as well as a buyer for their former home. As closing day approached, the couple had one task to complete. Each would need a certificate of sanity from a psychologist.

“They will look at our psychological state,” she explained. “If everything is normal, they will give us the document.”

Ms. Kotova, and tens of thousands of other Russians who are required to produce such a document to sell property, have no history of mental illness. The certificate, which must be signed and stamped by a doctor, is a legal defense against a pervasive problem in Moscow real estate deals: rampant fraud and weak courts.