The heavy shadow of modern portfolio theory (MPT) has had a massive impact on everything from market structure, investment philosophy, and investor behavior, to the research that examines those disciplines. Researchers believe that they are casting light onto investment issues (including, for this purpose, specifically investor time horizons), but generalized acceptance of MPT allows it to continue to darken what should be enlightened.
Jim Hawley and Jon Lukomnik, The Long and Short of It: Are We Asking the Right Questions? Modern Portfolio Theory and Time Horizons, 41 SEATTLE U. L. REV. 449 (2018).