In the present-day, different liberal democracies use the adversarial and inquisitorial legal systems (e.g., the U.S. and France, respectively). Have there been any comparative studies of whether either system is more prone to produce biased results in convictions and sentencing—for instance, results biased by racial or political factors?
Thibaut et al. (1972) conducted a roleplaying experiment to test whether presenting evidence using an adversarial or inquisitorial system would influence bias. They found that the system used did not influence unbiased subjects, but that the adversarial system was effective in counteracting initial bias.
Thibaut, John, Laurens Walker, and E. Allen Lind. 1972. "Adversary Presentation and Bias in Legal Decisionmaking". Harvard Law Review 86:2.
I do not have a direct source handy, but read this on a TVTropes useful notes article on Common Law. With all things considered equal, an Common Law (Adversarial) System has a very slight, if negligible tendency to find in favor of the defendant and thus release a possible guilty party. A Civil Law (Inquisitorial) has a very slight tendency to find in favor of the prosecution and jail a possible innocent party. Again, cannot stress this enough, this is so slight it is reasonable to say it doesn't factor into the call as to which system is unjust.
As for racial and political motivations factoring into biased, this is difficult to factor because Common Law and Civil Law countries will experience these factors differently based on regional biases. For example, it's reasonable to say that bot the United States court system and the German Court system have their histories of biased decision making in the 1930 and 40s, but the degree to which had the worst outcome for the victim of these biases is clearly (hopefully) frighteningly obvious.
There is possiblity to hypothetically show systemic bias control that may favor an Adversarial System where the Trier of Law (Asks "What needs to be proven to say a crime occurs?") and Trier of Fact (Asks "Does this specific case meet the criteria of the Law") are separate entities (The Judge and the Jury, respectively) where as the two decision makers are the same body in the Inquisitorial System (A Judge or panel of Judges, depending on specific system). This isn't to say that either question cannot be biased, but the Adversarial does can insulate biases in one question's answer from determining the answer to the second question. Additionally, Jury being the Trier of Fact mean that the more damning question has to be answered by 12 people unanimously. It is quite a difficult task of getting 12 people to agree to anything, even when presented with the same facts. However, the poison of biased decision making is present in both systems and has affected countries that practiced both systems throughout history, so this isn't a hard stance on the matter.