The legislative intent of crimes against the family is as follows:
944.01 Intent. The state recognizes that it has a duty to encourage
high moral standards. Although the state does not regulate the
private sexual activity of consenting adults, the state does not condone
or encourage any form of sexual conduct outside the institution
of marriage. Marriage is the foundation of family and society.
Its stability is basic to morality and civilization, and of vital interest
to society and this state.
Further, under Wisconsin law, if a married person has sexual intercourse with a person who is not his spouse, both parties commit the crime of adultery. Under Wisconsin law (WI Statute 944.16), adultery is a Class I felony.
The penalty for a Class I Felony is a fine of up to $10,000, or imprisonment of up to 3-1/2 years, or both; however, for a repeat offender, the term of imprisonment may increase up to 2 years with prior misdemeanor convictions, and up to 6 years with a prior felony conviction.
The Prosecuting Attorney must be able to prove all elements of the crime "beyond a reasonable doubt." The elements are: 1) the person must be married, and 2) the person has sexual intercourse with someone who is not his/her spouse.
You must take your evidence to the County Prosecutor's office and the local police department. Ultimately, however, it is the Prosecutor's office that will make the decision as to whether a criminal action may be brought. Therefore, any evidence that you have to build your case is important. Photographs, e-mails, letters, cards, videos, telephone logs, etc. are all very important. So, your thought about hiring a private investigator may be a very good one if you wish to pursue a criminal charge, and may assist you in negotiating your property settlement.
Although Wisconsin is a "no fault" state, that does NOT mean that you cannot allege adultry as one of (if not the only) reason for the divorce. There are certain allegations that must be made in all divorces, and adultry may be one of them. Please see the link below:
Also below is a link to the factors that the court will take into consideration for the division of your marital assets. Note, that the final factor is "all other relevant factors." Adultry is certainly a relevant factor. His misdeed may allow you to retain more of the marital assets than would otherwise be awarded.
Finally, below please find an article from the New York Times that discusses a 1990 case for adultry that was prosecuted in Wisconsin. I think (but not sure) the person was convicted.
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Legal Disclaimer: This is for informational purposes only and does not constitute legal advice.