I know the first part means that "no ethnic is inherently easier to form a gang than another ethnic, but being marginalized makes some ethnic groups have more gangs". I also know "account for" might be "to constitute the governing or primary factor in". But I don't know what the this in boldface refer to, and be account for what here? What does "be accounted for" mean?
No ethnic group is more disposed to gang involvement than another, rather it is the status of being marginalized, alienated or rejected that makes some groups more vulnerable to gang formation, and this would also be accounted for in the effect of social exclusion, especially in terms of recruitment and retention.