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El Nino-Southern Oscillation (ENSO) dominates interannual climate variability; thus, understanding its response to climate forcing is critical. ENSO's sensitivity to changing insolation is poorly understood, due to contrasting interpretations of Holocene proxy records. Some records show dampened ENSO during the early to mid-Holocene, consistent with insolation forcing of ENSO amplitude, but other records emphasize decadal-centennial fluctuations in ENSO strength, with no clear trend. To clarify Holocene ENSO behavior, we collected proxy data spanning the last similar to 12 kyr and find relatively low El Nino amplitude during the early to mid-Holocene. Our data, together with published work, indicate both a long-term trend in ENSO strength due to June insolation forcing and high-amplitude decadal-centennial fluctuations; both behaviors are shown in models. The best supported mechanism for insolation-driven dampening of ENSO is weakening of the upwelling feedback by insolation-forced warming/deepening of thermocline source waters. Elucidating the thermocline's role will help predict future ENSO change.