Main Effects & Interactions

Makeup & Attractiveness Experiment (Psych 321, Spring 2016)

Question: Does makeup increase perceived attractiveness?
Design: between-subjects 2 \times 1
- IV1: Makeup (present vs absent)
- IV2: Rater Gender (male, female)
- Stimuli: 5 identical female faces; attractiveness scale 1–10
Overall means (collapsed across gender)
- With makeup: 7.4
- Without makeup: 7.3 ⇒ no main effect of makeup
Split by gender
- Male raters: 7.9 (makeup) vs 7.3 (no makeup)
- Female raters: 6.8 (makeup) vs 7.3 (no makeup)
Conclusion: Significant Makeup × Gender interaction; effect of makeup depends on rater gender (↑ for males, ↓ for females)

Main effect: consistent influence of an IV on the DV after averaging over other IVs
Interaction: effect of one IV changes across levels of another IV
\text{"Effect of IV}1\text{ depends on IV}2" ⇒ answers become “it depends”

Question: Does hands-free phone use impair braking, and does this vary with age?
Design: 2 \times 2 between-subjects
- IV1: Phone Use (hands-free vs none)
- IV2: Age Group (Younger 18–25 vs Older 65–74)
- DV: Brake-onset time (ms) after obstacle appears (lower = better)
Findings
- Main effect of Phone: Phone \approx1000 ms vs No phone \approx850 ms
- Main effect of Age: Older > Younger in all conditions
- No Phone × Age interaction: phone slows both age groups by similar margin
Notable: Younger drivers on phone ≈ braking speed of older drivers off phone (functional “aging” of \sim50 years)

Examine graph/means:
- Non-parallel lines → possible interaction
- Vertical gap between averaged lines → main effect
Reporting order: each main effect (significant/not) → interaction (significant/not)
Interpretation guide:
\text{Interaction significant} \Rightarrow describe pattern within each subgroup; main effects may mislead
\text{No interaction} \Rightarrow main effects generalize across other IV