First, the variable 
  
    
      
        
          x
          
            j
          
        
      
    
    {\displaystyle x_{j}}
   may be a non-economic variable: for example, if rainfall amount 
  
    
      
        
          x
          
            j
          
        
      
    
    {\displaystyle x_{j}}
   is hypothesized to affect the futures price y of some agricultural commodity, it is impossible that in fact the futures price affects rainfall amount (provided that cloud seeding is never attempted). Second, the instrumental variables technique may be employed to remove any reverse causation by introducing a role for other variables (instruments) that are known to be unaffected by the dependent variable.
Trauma to the hymen may heal without any visible sign of injury. An observational study of adolescent sexual assault victims found that majority of wounds to the hymen healed without any visible sign of injury having occurred.Trauma to the hymen is hypothesized to occur as a result of various other behaviors, such as tampon or menstrual cup use, pelvic examinations with a speculum, masturbation, gymnastics, or horseback riding, although the true prevalence of trauma as a result of these activities is unclear. Cultural significance
The hymen is often attributed important cultural significance in certain communities because of its association with a womans virginity. In those cultures, an intact hymen is highly valued at marriage in the belief that this is a proof of virginity. Some women undergo hymenorrhaphy to restore their hymen for this reason.In October 2018, the UN Human Rights, UN Women and the World Health Organization (WHO) stated that virginity testing must end as "it is a painful, humiliating and traumatic practice, constituting violence against women". Womb fury
In the 16th and 17th centuries, medical researchers mistakenly saw the presence or absence of the hymen as founding evidence of physical diseases such as "womb-fury", i.e., (female) hysteria. If not cured, womb-fury would, according to doctors practicing at the time, result in death. Other animals
Due to similar reproductive system development, many mammals have hymens, including chimpanzees, elephants, manatees, whales, horses and llamas. See also
Artificial hymen

References
External links

Magical Cups and Bloody Brides—the historical context of virginity
20 Questions About Virginity—Interview with Hanne Blank, author of Virgin: The Untouched History.