Русско-англо-русский юридический словарь

Mitigating Circumstances Information about the defendant that does not justify or excuse a criminal act or civil wrong, but that may reduce the defendant's degree of culpability. In criminal law, juries consider mitigating circumstances when deciding whether to impose the death sentence in a capital case, and judges may consider them when selecting a sentence. In civil cases, mitigating circumstances may reduce the amount of damages awarded to the plaintiff. (Nolo's Plain-English Law Dictionary)

mitigating circumstance Accompanying or accessory condition, event, or fact that (though not constituting a justification or excuse of an offense) may be considered by the courts as reducing the degree of culpability or liability of the accused. Such circumstances may include family or personal situations, and may help in attracting a sentence less severe than a typical sentence for similar offenses. (BusinessDictionary.com)

All in all it would seem that we don't need a change as long as the jury understand these factors and handing down a guilty verdict on all concerned leaves the judge to decide the degree of culpability for each defendant and set their sentences appropriately. (BBC - comment)

The two principal determinants of a sentence remain the degree of harm the drug would inflict and the degree of culpability of the offender. (The Guardian)