Table 2.

Recurrence Rate as a function of “recurrence” definition.

Recurrence Definition	Number of siblings affected by definition		Proportion(%) of families with additional affected sib	Proportion (%) of all siblings in the families affected
	Male	Female
Single Criterion
Categorical ASD DX	100	38*	10.9**	8.2
SCQ > =15	108	53	12.4	9.6
SRS-T >= 75	99	64*	12.5	9.7
History of Language Delay with Autistic Speech	80	70	10.8	8.9
Category “Change” when switching from categorical to quantitative threshold
No ASD DX but SCQ>=15	27	22	--	--
No ASD DX but SRS-T>=75	28	33	--	--
Combined Criteria
ASD or SCQ>=15	127	60	14.4	11.1
ASD or SRS-T>=75	128	71	15.1	11.8
ASD or History of Language Delay with Autistic Speech	180	108	21.7**	17.1
ASD or History of Language Delay with Autistic Speech or SCQ>=15 or SRS-T>=75	211	142	26.0	20.9

When exclusively considering later-born siblings (of relevance for comparison to high-risk infant sibling studies), the proportion of individual siblings affected by ASD was 14.2%. When including history of language delay with autistic speech and ASD in the definition of affected status, that proportion was 23.2%.

^*For difference in proportion of all female sibs affected as a function of recurrence classification, comparing standardized T-score threshold to community diagnosis, McNemar’s Test, Two-Tail p=.000042

^**Difference in recurrence rate with versus without inclusion of siblings with History of Language Delay with Autistic Speech, p<.001.