There is no known effect, a fact now proven by very reputable and extensive studies. There were a few junk science studies published years ago, debunked many times hence, yet the myth persists because scaremongers have found a way to make money by stirring up fears and reciting these studies without mentioning that they have need disproved.
Anecdotally, I have lived under 250kV power lines for 18 years now and raised two children here for much of their lives with no ill effects on any of us. Many of my neighbors have lived here since the housing development was built in 1970, nobody has reported any effects whatsoever. Yet, whenever a house goes up for sale in this neighborhood, the realtors make the rounds wanting to know who around here got cancer, who suffers from psychological disorders etc etc. They almost seem disappointed in our answers, because I think they are looking for reasons to de-value the properties and sell them faster. So far, they have been out of luck.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington