Verbs of Reference

Referring verbs use both the present and the past tenses. It is probably best to use the present tense for recent sources or when you feel that the idea or data is still valid:

Rathbone (1997) demonstrates the limitations of video-conferencing.

The past tense suggests that the source is older and the ideas perhaps out of date:

Steinbeck (1965) explored a link between cancer and diet.