TABLE 1.

Descriptive statistics of general cognitive ability and temperament factors

Age in years	N	MZ/DZ pairs	General cognitive ability Mean (SD)	Persistence Mean (SD)	Approach Mean (SD)	Reactivity Mean (SD)
4	356	83/94	90.80 (14.94)	72.57 (12.59)	92.62 (18.27)	125.21 (15.04)
5	351	82/93	95.57 (14.24)	69.65 (13.06)	91.09 (19.46)	123.80 (15.98)
6	353	87/89	98.27 (13.03)	67.86 (12.27)	87.20 (19.47)	123.24 (16.78)
7	222	50/61	97.16 (14.50)	67.27 (12.15)	86.21 (20.21)	121.23 (18.25)
8	273	70/66	99.83 (14.14)	62.90 (9.65)	92.60 (13.22)	109.96 (10.76)
9	270	69/66	102.06 (14.47)	62.70 (9.74)	92.34 (14.02)	110.10 (9.40)

Note: Higher scores on the temperament factors indicate more difficulties.

Abbreviations: DZ, dizygotic twins; MZ, monozygotic twins.