The principle of total evidence (PTE) asserts that rational agents should conditionalize their degrees of belief on the total evidence that is accessible to them. In the first part of this talk I will justify the optimality of the PTE from the viewpoint of individual rationality, in the sense of expected utility maximization. My proposed justification of the PTE is based on a seminal proof of Good (1967) which shows that by conditionalizing degrees of beliefs on new pieces of evidence or information, the expected individual utility of the actions of a rational agent can only increase but not decrease.

In the second part I will demonstrate that in spite of its individual optimality, the PTE may become suboptimal for the collective rationality when interests are in conflict. This scenario can arise when the acquiration of a certain piece of information by a rational agent carries a great risk of being "misused" in a way that, in spite of being useful for the individual, causes great harm to others and, thus, decreases the overall utility-sum (or aggregated utility) for the whole collective.