People worldwide are faced with making decisions daily. While many decisions are quick (e.g., what clothes to wear), others, such as those about environmental issues (e.g., overfishing), require more thought and have less immediate outcomes. How one makes such decisions depends on how one interprets, evaluates, and uses evidence. The central objective of this thesis was to investigate environmental science literacy in general, and specifically, to understand how evidence and other factors impact decision-making. I conducted three main studies:

First, I provide an example of how decision-making practices affect environmental systems and services through a descriptive case study of Atlantic bluefin tuna overfishing. I reviewed the scientific, historical and cultural factors contributing to a paradox of marine preservation in the Mediterranean and highlighted the need for education and informed decision-making about such social and ecological issues. This study motivated me to investigate how people make decisions about environmental issues.

Second, I interviewed middle school students to understand how they describe and evaluate evidence hypothetically and in practice about environmental issues---a key component of environmental literacy. Students discussed how they would evaluate evidence and then were then given a packet containing multiple excerpts of information from conflicting stakeholders about an environmental issue and asked how they would make voting or purchasing decisions about these issues. Findings showed that students' ideas about evaluating evidence (e.g., by scientific and non-scientific criteria) match their practices in part. This study was unique in that it investigated how students evaluate evidence that (1) contradicts other evidence and (2), conflicts with the student's prior positions.

Finally, I investigated whether middle school students used evidence when making decisions about socioscientific issues. I hypothesized that holding a strong opinion would decrease the likelihood of changing decisions when presented with additional information. Findings indicated that most students do not change their stance after reading additional evidence. Students were more likely to change their decisions about issues that they cared least about than about issues that they cared most about. Implications for science teaching and learning are discussed.