The increased demand for labor in the Deep South pushed up the price of slaves in markets such as New Orleans, which became the fourth-largest city in the country based in part on profits from the slave trade and related businesses. The price differences between the Upper and Deep South created demand. Slave traders took advantage of this arbitrage opportunity by buying at lower prices in the Upper South and then selling slaves at a profit after taking or transporting them further south. Some scholars believe there was an increasing prevalence in the Upper South of "breeding" slaves for export. The proven reproductive capacity of enslaved women was advertised as selling point and a feature that increased value.