Rudd and Herzberger report that brothers who committed incest were more likely to use force than fathers who commit incest (64% vs. 53%). Similarly, Cyr and colleagues found that about 70% of sibling incest involved sexual penetration, substantially higher than other forms of incest. Bass and colleagues write that "sibling incest occurs at a frequency that rivals and may even exceed other forms of incest," yet only 11% of studies into child sex abuse examined sibling perpetrators. Rayment and Owen report that "comparison of the offending patterns of sibling offenders with other teenage sex offenders ... Sibling abusers admitted to more sexual offences, had a longer offending history and a majority engaged in more intrusive sexual behaviour than other adolescent sex offenders. The sibling perpetrator has more access to the victim and exists within a structure of silence and guilt." A survey of eight hundred college students reported by David Finkelhor in "Journal of Marriage and Family Counseling" found that fifteen percent of females and ten percent of males had been sexually abused by an older sibling.