  The stellar initial mass function (IMF) describes the distribution in stellar
masses produced from a burst of star formation. For more than fifty years, the
implicit assumption underpinning most areas of research involving the IMF has
been that it is universal, regardless of time and environment. We measure the
high-mass IMF slope for a sample of low-to-moderate redshift galaxies from the
Galaxy And Mass Assembly survey. The large range in luminosities and galaxy
masses of the sample permits the exploration of underlying IMF dependencies. A
strong IMF-star formation rate dependency is discovered, which shows that
highly star forming galaxies form proportionally more massive stars (they have
IMFs with flatter power-law slopes) than galaxies with low star formation
rates. This has a significant impact on a wide variety of galaxy evolution
studies, all of which rely on assumptions about the slope of the IMF. Our
result is supported by, and provides an explanation for, the results of
numerous recent explorations suggesting a variation of or evolution in the IMF.
