Table 7.

Contribution of additive genetic effects (A), common environment (C), and unique environment (E) to variance in percent and absolute mammographic density

Strata of twinsnPercent mammographic density
Absolute mammographic density
A (SE)*C (SE)*COV*EPA (SE)*C (SE)*COVEP
All twins (adjusted for age only)1,1060.75 (0.02)00.020.230.63 (0.10)0.07 (0.09)0.020.29
All twins (adjusted for multiple factors)§1,1060.53 (0.09)0.01 (0.08)0.200.250.59 (0.10)0.08 (0.09)0.060.28
Both alcohol use ≤2 drinks/mo§3180.65 (0.03)00.170.180.84 (0.02)00.040.12
Both alcohol use >2 drinks/mo§2300.53 (0.06)00.200.270.67 (0.06)00.080.26
Alcohol use discordant§3120.53 (0.16)0.02 (0.14)0.220.240.200.08 (0.23)0.39 (0.17)0.050.48<0.0001
Alcohol use unknown for at least one twin§2460.24 (0.19)0.21 (0.16)0.230.310.25 (0.17)0.40 (0.15)0.080.26
BMI difference <1.1§2900.34 (0.16)0.28 (0.15)0.160.220.57 (0.19)0.21 (0.18)0.070.16
BMI difference ≥1.1§8160.51 (0.03)00.220.270.0250.47 (0.12)0.11 (0.10)0.070.350.0002
BMI difference ≤2.5§6000.47 (0.11)0.15 (0.11)0.170.210.69 (0.12)0.12 (0.12)0.030.17
BMI difference >2.5§5060.42 (0.06)00.220.360.00090.20 (0.19)0.21 (0.14)0.100.490.0001
Menarche within 1 y§4280.51 (0.04)00.180.300.71 (0.03)00.070.23
Menarche >1 y apart§6780.57 (0.10)0.01 (0.09)0.220.190.0030.48 (0.13)0.14 (0.11)0.070.320.0004
Both nulliparous§3800.45 (0.09)0.330.2200.80 (0.04)0.100.10
Both parous§8020.47 (0.10)0.07 (0.09)0.190.270.62 (0.11)0.07 (0.10)0.070.24
Discordant for nulliparity§2660.51 (0.05)00.250.230.0040.53 (0.07)00.090.390.0006
Both 1-2 births§2180.19 (0.15)0.34 (0.13)0.230.240.43 (0.18)0.22 (0.16)0.110.23
Both ≥3 births§2180.51 (0.19)0.03 (0.17)0.210.250.70 (0.05)00.110.20
Both parous, discordant 1-2 vs ≥3 births§3660.54 (0.05)00.190.270.0090.64 (0.17)0.06 (0.15)0.070.230.004
Both premenopausal§2940.57 (0.05)00.180.250.48 (0.18)0.23 (0.16)0.010.28
Both postmenopausal§4560.51 (0.14)0.03 (0.12)0.190.270.44 (0.16)0.20 (0.14)0.040.32
Discordant§2160.66 (0.17)0.04 (0.17)0.150.160.0020.75 (0.06)00.010.25<0.0001
At least one twin unknown1400.40 (0.10)00.230.370.70 (0.08)00.050.25
Menopausal pairs§
    Both current HT users§2320.33 (0.20)0.20 (0.17)0.170.310.42 (0.25)0.16 (0.21)0.010.40
    Both nonusers§6000.68 (0.06)0.150.1700.69 (0.09)0.050.26
    Discordant§1380.49 (0.09)00.270.240.00060.75 (0.27)0.02 (0.27)0.090.130.0002
    At least one twin unknown260.38 (0.24)00.240.380.37 (0.24)00.200.43
Close as adults§7100.45 (0.11)0.09 (0.10)0.210.250.62 (0.12)0.08 (0.12)0.050.24
Not close as adults§3960.55 (0.05)00.210.250.150.40 (0.19)0.15 (0.15)0.080.380.053
  • * Proportion of total variance due to A = additive genetic effects, C = common environment, and COV = covariates as estimated by taking the proportion of residual variance due to A or C and the proportion of variance due to covariates over the total variance (1 + proportion of variance due to covariates). C was automatically dropped from model in those instances it was estimated to be equal to 0.

  • Proportion of total variance due to E = individual environment as inferred from the equation A + C + COV + E = 1.

  • P value from likelihood ratio test comparing the log likelihood of model with all twins combined to models allowing the estimates to vary by stratum. Unknown categories were excluded from P value calculations.

  • § Model adjusted for the following covariates: BMI (continuous), age at menarche (continuous), age at mammogram (continuous), parity (continuous), and menopausal status/hormone use.