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. Author manuscript; available in PMC: 2010 Mar 24.
Published in final edited form as: Arch Gen Psychiatry. 2008 Jun;65(6):674–682. doi: 10.1001/archpsyc.65.6.674

Table. Similarity of the Peer Group and of Nicotine Use in Monozygotic and Dizygotic Twins Over Development.

Frequency of Same Friends, Meana Correlation in Nicotine Use
Age, y MZ DZ MZ-DZ MZ DZ MZ-DZ
12-14 3.19 2.66 0.53 0.34 0.34 0.00
15-17 2.89 2.31 0.58 0.52 0.47 0.07
18-21 2.39 1.89 0.50 0.55 0.37 0.18
22-25 2.16 1.76 0.40 0.55 0.31 0.24

Abbreviations: DZ, dizygotic; MZ, monozygotic.

a

1 indicates never; 2, sometimes; 3, usually; and 4, always.