Allport’s four contact conditions must operate together, not individually.

Evidence

Consequently, we employed two global indicators of Allport’s proposed conditions – intergroup friendship and structured programs for optimal contact. These ratings actually offer more direct tests of the theory than our original approach, since Allport held that his four conditions should be integrated and implemented together, rather than listing them as variables to be considered individually. In addition, these two indices offer a stringent test of Allport’s contentions, because they are negatively related to each other (p < 0.001). Indeed, only 4 of the 134 samples that experienced optimal structured contact used friends as the measure of contact. — John F. Dovidio, Peter Glick, Laurie A. Rudman, On the Nature of Prejudice, 1692