Presumption of innocence and in dubio pro reo: two guarantees that are not the same
Two of the most important principles of criminal proceedings are also two of the most confused. The presumption of innocence and the principle in dubio pro reo are distinct concepts, operating at different moments and producing different effects, even though in everyday language they are used as synonyms. Understanding the difference explains why many trials end in acquittal and why the burden of proof falls on the prosecution.
The presumption of innocence is a fundamental right recognised in Article 24.2 of the Constitution, in the European Convention on Human Rights and in the Universal Declaration. It means that every accused person must be considered innocent until their guilt is established by a final judgment. It operates in three simultaneous dimensions: as a rule of treatment, the accused cannot be treated as guilty before conviction; as a rule of evidence, guilt must be established by the prosecution with sufficient incriminating evidence obtained with all the guarantees, without the defence having to prove innocence; and as a rule of judgment, if in the end a reasonable doubt persists, the court must acquit.
From this derives the burden of proof: whoever accuses must prove, and the accused does not have to demonstrate that they are innocent. If the prosecution does not provide sufficient evidence to establish guilt beyond all reasonable doubt, the court must acquit, even if the accused has provided no evidence at all or their account is not entirely convincing. This rule responds to the asymmetric cost of error: convicting an innocent person is far more serious than acquitting a guilty one.
The required standard is certainty beyond all reasonable doubt: not an absolute certainty, impossible to attain, but something more demanding than mere probability. A reasonable doubt is one that, based on reason and common sense, has an objective foundation.
The principle in dubio pro reo, which means when in doubt, in favour of the accused, is a rule of evidentiary assessment that operates in the final phase of the process: when the court, having examined all the evidence, faces the uncertainty that it points to guilt but without sufficient force. At that moment, the doubt must be resolved in favour of the accused. Unlike the presumption of innocence, which is a fundamental right with constitutional rank, in dubio pro reo is a procedural rule that complements and gives concrete shape to it.
The essential differences are three. The timing: the presumption of innocence operates throughout the process; in dubio pro reo, only in the final assessment of the evidence. The nature: the first is a fundamental right whose infringement can be taken to an appeal for protection (amparo); the second, a procedural rule that can be invoked in cassation. And the scope: the presumption of innocence is infringed by convicting without sufficient incriminating evidence; in dubio pro reo, by convicting despite acknowledging reasonable doubts.
Both act in a complementary way. For the defence, the strategic key is not always to prove innocence, but to generate reasonable doubts about the prosecution's account: to identify gaps in the chain of evidence, question the reliability of witnesses and offer alternative explanations. If those doubts are reasonable, the court must acquit.
