▁to ▁describe ▁the ▁effect ▁and ▁season ▁of ▁birth ▁bias ▁is ▁used ▁to ▁describe ▁similar ▁effects ▁driven ▁by ▁different ▁hypoth es ised ▁mechan isms . ▁ ▁The ▁bias ▁results ▁from ▁the ▁common ▁use ▁of ▁age ▁related ▁systems , ▁for ▁organ izing ▁youth ▁sports ▁competition ▁and ▁academic ▁coh ort s , ▁based ▁on ▁specific ▁cut - off ▁dates ▁to ▁establish ▁el ig ibility ▁for ▁inclusion . ▁Typ ically ▁a ▁child ▁born ▁after ▁the ▁cut - off ▁date ▁is ▁included ▁in ▁a ▁coh ort ▁and ▁a ▁child ▁born ▁before ▁the ▁cut - off ▁date ▁is ▁ex cluded ▁from ▁it . ▁ ▁In ▁sports ▁ ▁The ▁most ▁commonly ▁used ▁cut - off ▁date ▁for ▁youth ▁international ▁sport ing ▁competition ▁is ▁ 1 ▁January . ▁The ▁IO C ▁and ▁FIFA ▁and ▁the ▁ 6 ▁international ▁football ▁conf eder ations ▁( AF C , ▁C AF , ▁CON C AC AF , ▁CON ME B OL , ▁OF C ▁and ▁UEFA ) ▁all ▁use ▁ 1 ▁January ▁as ▁their ▁administrative ▁cut - off ▁date ▁when ▁determ ining ▁an ▁ath lete ' s ▁el ig ibility ▁to ▁comp ete ▁in ▁youth ▁compet itions , ▁children ▁born ▁before ▁a ▁specified ▁cut - off ▁date ▁are ▁ex cluded . ▁ ▁Mal col m ▁Glad well ' s ▁book ▁Out liers : ▁The ▁Story ▁of ▁Success ▁and ▁Super Fre ak onom ics ▁by ▁Steven ▁Lev itt ▁and ▁Stephen ▁Dub ner ▁have ▁popular ised ▁the ▁issue ▁in ▁respect ▁of ▁Canadian ▁ice - ho ck ey ▁players , ▁European ▁football ▁players ▁and ▁US ▁Major ▁League