This paper offers a number of refinements to Jarvis's (2000) methodological framework for investigating cross-linguistic effects. According to the original framework, there are three potential consequences of cross-linguistic effects, and any compelling argument for or against the presence of such effects must be based on a consideration of all three consequences. These consequences can be thought of as types of evidence, or premises for transfer, and their investigation requires performance comparisons between individuals, groups, and languages. The present paper has two purposes. The first is to characterize the foundations of a classification scheme that highlights the relationships among these three types of evidence and also indicates that there is yet a fourth type of evidence for cross-linguistic effects that has not yet been taken account of within the framework. The second purpose of this paper is to show that these four types of evidence constitute what can be described as a comparison-based argument for transfer, but that there also exist other valid arguments for transfer, such as what I refer to as the detection-based argument. The paper describes both the comparison- and detection-based arguments and the types of evidence they entail in relation to number of recent investigations into cross-linguistic influence.