Abstract
This study examines the effect of merger momentum on acquirer's returns both in the short and long-run. The focus is on high valuation markets and the source of momentum is investigated employing three different hypotheses: the neoclassical hypothesis, the hubris hypothesis and the investor sentiment theory. Evidence is provided that supports the investor sentiment (optimism) hypothesis since it is demonstrated that investors earn significant gains in the short run but returns are reversed in the long-run as initial expectations may not be fully met when combined firms’ accomplishments become known over time. The results are robust after controlling for several acquirer and deal characteristics