We have made a thorough study of the low-energy behaviour of the $gamma$-ray strength function within the framework of the shell model. We have performed large-scale calculations spanning isotopic and isotonic chains over several mass regions, with the purpose of studying the systematic behavior of the low-energy enhancement (LEE) for $M1$ transitions. There are clear trends in the calculations: From being all but absent in the lowest mass region, the LEE becomes steeper and more pronounced as the mass number increases, and for a given mass region it further increases towards shell closures. Moreover, the LEE is found to be steeper in regions near doubly-magic nuclei where proton particles couple to neutron holes. These trends enable us to consolidate several previous works on the LEE into a single, consistent concept. We compare the inferred trends to the available experimental data from the Oslo method, and find suppport for the systematic behaviour. Lastly we have compared the calculations to strength functions compiled from discrete, experimental lifetimes, and find excellent agreement; the discrete data are consistent with a LEE, and indicate that the slope varies as function of mass number.
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