Solange's secret is one she shared with thousands of women in Europe until the 1970s. These women were strangled by shame. They could not talk about their secret. Society would not let them anyway, as they were considered outlaws. These women had aborted.
This is the very serious issue "What have you done to Solange?" was built on back in 1972.
Living in Italy or in France, you only had two options: either leave the country to have a legal abortion in a hospital in the UK or the Netherlands. Or abort in clandestinity - which usually meant dreadful conditions, leaving women wounded forever or even killing them (on this particular point it is noticeable that Solange Beauregard is probably French and that the film takes place in England, shot by an Italian director).
Contrary to a lot of gialli, here the women are somehow empowered: only them have the answer to the question raised in the title. Men don't know or don't understand. It's a women's secret.
But they are the victims as well. They form a shadow society which members are tied together by a common psychological and physical suffering. Solange's pain left her muted for ever. Wounded in such a way that she will never become a real woman. And neither revolve to be a child again.
Worse, the film demontrates she suffers in vain: by taking a revenge on the women Solange was the victim of, the killer spreads the barbary even further. Dallamano even manages to underline the ambiguous position of the Church, questioning its role as in other gialli, by showing the murderer dressing up as a priest to gain his preys' trust and have them revealing their terrible secrets.
Silence, secrets, taboo are the true killers of this desperate movie.
In this instance, "Solange" is a highly political giallo: it was urging everybody to bring the abortion issue to light and debate it publicly.
In France, abortion was legalized in 1974. In Italy, in 1978.
More on "What have you done to Solange?" here.