Twitter made an important announcement last week: that it was adding a woman, Marjorie Scardino, to its board. This is significant because Twitter has become a poster child for the sexism and arrogance that pervades Silicon Valley. This appointment sends a signal to the corporate world that behavior that was considered acceptable in the 1950s is no longer so — that there must be diversity.

Scardino is a great choice for Twitter's board. She is former chief executive of publishing giant Pearson and is highly accomplished. She has been an outspoken critic of the lack of diversity on boards and is likely to speak up when she sees frat-boy behavior. But we can't let Twitter off the hook. It has garnered worldwide attention for bad corporate governance practices. So, it must now be the example of what is good. It should create a more balanced board, management team and employee base, with more women as well as African-Americans, Hispanics and Asians in the mix. It needs to do this because it is good for the company.

Adding more women as board members will improve the financial performance of the company. As research by Catalyst shows, companies with the highest proportions of female board directors outperform those with the lowest proportions by 53 percent. They have a 42 percent higher return on sales and 66 percent higher returns on invested capital.