Table 4.
Parameter Estimates and Fit Statistics from the Unconstrained and Best-Fitting Twin Models for physical activity.
| Standardized Parameter Estimates | Fit Statistics | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Girls | Boys | ||||||||||
| Model | A | C | E | A | C | E | −2lnL (df) | χ2(df)a | Δχ2 (df)b | p | AIC |
| Physical Activity | |||||||||||
| Baseline | -- | -- | -- | -- | -- | -- | 650.84 (249) | -- | -- | -- | -- |
| ACE, unconstrained | 0 (0, 0) |
.34 (0, .50) |
.66 (.50, .85) |
0 (0, 0) |
.65 (.13, .80) |
.35 (.20, .57) |
680.50 (261) | 29.66 (12) | -- | -- | 5.66 |
| ACE, constrained to be equal across sex | 0 (0, .15) |
.51 (.33, .63) |
.49 (.37, .63) |
0 (0, .15) |
.51 (.33, .63) |
.49 (.37, .63) |
712.60 (264) | 61.76 (15) | 32.10 (3) | <.01 | 31.76 |
| CE, constrained to be equal across sex | -- | .51 (.37, .63) |
.49 (.37, .63) |
-- | .51 (.37, .63) |
.49 (.37, .63) |
712.60 (265) | 61.76 (13) | 32.10 (1) | <.01 | 35.76 |
| CE, A constrained to 0 | -- |
.33
(.14, .49) |
.67 (.51, .86) |
-- |
.66 (.44, .81) |
.34 (.19, .56) |
680.50 (263) | 29.66 (14) | 0.00 (2) | .99 | 1.66 |
Note. The unconstrained ACE model allows all genetic (A) shared environmental (C), and nonshared environmental (E) influences were allowed to vary across sex. Unless specified, models allow A, C, and E to differ across sex. Parameter estimates are standardized and range from −1.0 to 1.0. Age was regressed out of both variables prior to model fitting. The best-fitting models are indicated by boldface. Ninety-five percent confidence intervals are in parentheses. 2lnL = −2 times log likelihood of data; df = degrees of freedom; AIC = Akaike’s information criterion.
χ2 is computed by subtracting the-2lnL of the baseline model from the −2lnL of the submodel; the AIC is calculated using this fit statistic (i.e., AIC = χ2 – 2*df)
Δχ2 compares submodels to the full ACE model