New Delhi, Aug. 1 -- NEW YORK - The moment arrives in most modern relationships, eventually. The bedroom light dims. One or both people reach for their phones instead of each other. Fifteen minutes later, half an hour, the screen still glows. The impulse that might have been there at the start of the evening has quietly retreated.

A study published in June in BMC Psychology now offers a formal explanation for what many couples have been sensing without the vocabulary to name it. Social media addiction, researchers found, is meaningfully associated with lower sexual desire. The more addicted to social platforms a person scores, the less they report wanting sex.

The research, led by Andrea Varaona-Santos and colleagues in Spain, used an o...