▁US ▁law , ▁the ▁term ▁illegal ▁per ▁se ▁means ▁that ▁the ▁act ▁is ▁inher ently ▁illegal . ▁Thus , ▁an ▁act ▁is ▁illegal ▁without ▁extr insic ▁proof ▁of ▁any ▁surrounding ▁circumstances ▁such ▁as ▁lack ▁of ▁sci enter ▁( know ledge ) ▁or ▁other ▁def enses . ▁Act s ▁are ▁made ▁illegal ▁per ▁se ▁by ▁stat ute , ▁constitution ▁or ▁case ▁law . ▁ ▁Dr unk ▁driving ▁Many ▁dr unk ▁driving ▁laws ▁make ▁driving ▁with ▁a ▁blood ▁alco hol ▁content ▁over ▁a ▁certain ▁limit ▁( such ▁as ▁ 0 . 0 5 % ▁or ▁ 0 . 0 8 %) ▁an ▁act ▁which ▁is ▁illegal ▁per ▁se . ▁ ▁Ant it rust ▁In ▁the ▁United ▁States , ▁illegal ▁per ▁se ▁often ▁refers ▁to ▁categories ▁of ▁anti - comp et itive ▁behavior ▁in ▁ant it rust ▁law ▁conclus ively ▁pres umed ▁to ▁be ▁an ▁" un reason able ▁rest raint ▁on ▁trade " ▁and ▁thus ▁anti ▁compet itive . ▁ ▁The ▁United ▁States ▁Supreme ▁Court ▁has , ▁in ▁the ▁past , ▁determined ▁activities ▁such ▁as ▁price ▁fixing , ▁ge ographic ▁market ▁division , ▁and ▁group ▁boy c ott ▁to ▁be ▁illegal ▁per ▁se ▁regardless ▁of ▁the ▁reason ab len ess ▁of ▁such ▁actions . ▁Trad itionally , ▁illegal ▁per ▁se ▁anti - tr ust ▁acts ▁describe ▁horizontal ▁market ▁arrang ements ▁among ▁compet itors . ▁ ▁The ▁illegal ▁per ▁se ▁category ▁can ▁trace ▁its ▁orig ins ▁in ▁the ▁ 1 8 9 8 ▁Supreme ▁Court ▁case ▁Add yst on ▁Pi pe ▁& ▁Ste el ▁Co .