Act more favorably. Want to appear intelligent, competent, stable. Participation can increase because of novelty or diminished because they have found out they were not selected. Some argue, most notably Richard Snow, that educational research is too artificial and lacks generalizability. This reflects both real-life environments and learning characteristics of students.
Environment is complex and interrelated.
People are active processors of information, do not react passively. When appropriate conduct research in natural setting. Incorporate several environmental variations. Watch what exactly students are doing during experiment. Observe social context of study.