Helen Fisher, a biological anthropologist at Rutgers University, is the source most commonly cited for the "18 months to 3 years" figure. Her research examines the neurochemistry of early-stage, passionate love — the dopamine-driven infatuation phase — and finds that this intense biological high typically subsides within that window as the brain habituates to the stimulus of a new partner.
Crucially, Fisher's own work does not claim that romantic love itself expires in three years. She distinguishes between three separate brain systems: lust, attraction (early-stage romantic love), and attachment. The fading of the attraction phase does not mean love ends — it can transition into a deeper attachment or, in some couples, sustain its early intensity. Fisher co-authored MRI research, highlighted by Harvard Medical School, showing couples married an average of 21 years displaying the same dopamine-rich brain activity as newly in-love individuals.
The American Psychological Association explicitly notes that romantic love can last a lifetime, citing longitudinal and neuroimaging studies. The popular shorthand attributing a hard expiration date to romantic love misrepresents Fisher's nuanced findings, which describe the trajectory of one phase of love — not the whole of it.