  This work investigates the main mechanism(s) that regulate the specific star
formation rate (SSFR) in nearby galaxies, cross-correlating two proxies of this
quantity -- the equivalent width of the \Ha\ line and the $(u-r)$ colour --
with other physical properties (mass, metallicity, environment, morphology, and
the presence of close companions) in a sample of $\sim82500$ galaxies extracted
from the Sloan Digital Sky Survey (SDSS). The existence of a relatively tight
`ageing sequence' in the colour-equivalent width plane favours a scenario where
the secular conversion of gas into stars (i.e. `nature') is the main physical
driver of the instantaneous SSFR and the gradual transition from a `chemically
primitive' (metal-poor and intensely star-forming) state to a `chemically
evolved' (metal-rich and passively evolving) system. Nevertheless,
environmental factors (i.e. `nurture') are also important. In the field,
galaxies may be temporarily affected by discrete `quenching' and `rejuvenation'
episodes, but such events show little statistical significance in a
probabilistic sense, and we find no evidence that galaxy interactions are, on
average, a dominant driver of star formation. Although visually classified
mergers tend to display systematically higher EW(H$\alpha$) and bluer $(u-r)$
colours for a given luminosity, most galaxies with high SSFR have uncertain
morphologies, which could be due to either internal or external processes.
Field galaxies of early and late morphological types are consistent with the
gradual `ageing' scenario, with no obvious signatures of a sudden decrease in
their SSFR. In contrast, star formation is significantly reduced and sometimes
completely quenched on a short time scale in dense environments, where many
objects are found on a `quenched sequence' in the colour-equivalent width
plane.
