The relevance of code comprehension in a developer's daily work was recognized more than 40 years ago. Over the years, several studies have gathered evidence that developers do indeed invest a considerable amount of their daily work in code comprehension. Consequently, many studies were conducted to find out how developers could be supported during code comprehension and which code characteristics contribute to better comprehension. Today, such experiments are more common than ever. While this is great for advancing the field, the number of publications makes it difficult to keep an overview. Additionally, designing rigorous experiments with human participants is a challenging task, and the multitude of design decisions and options can make it difficult for researchers to select a suitable design. We therefore conducted a systematic mapping study of 95 source code comprehension experiments published between 1979 and 2019. By systematically structuring the design characteristics of code comprehension studies, we provide a basis for subsequent discussion of the huge diversity of design options in the face of a lack of basic research on their consequences and comparability. We describe what topics have been studied, as well as how these studies have been designed, conducted, and reported. Frequently chosen design options and deficiencies are pointed out. We conclude with five concrete action items that we as a research community should address moving forward to improve publications of code comprehension experiments.