Table:
|  |  | Three Months Ended |  | % Variation |  |
| --- | --- | --- | --- | --- | --- |
|  | December 31, 2019 | September 29, 2019 | December 31, 2018 | Sequential | Year-Over-Year |
|  |  |  | (Unaudited, in millions) |  |  |
| Selling, general and administrative expenses | $(285) | $(267) | $(285) | (6.3)% | 0.4% |
| Research and development expenses | (387) | (362) | (345) | (7.0) | (12.3) |
| Total operating expenses | $(672) | $(629) | $(630) | (6.7)% | (6.6)% |
| As percentage of net revenues | (24.4)% | (24.7)% | (23.8)% | +30 bps | -60 bps |
Paragraphs:
The amount of our operating expenses increased by $43 million on a sequential basis, mainly driven by seasonality and salary dynamic.
On a year-over-year basis, our operating expenses increased by $42 million, mainly due to salary dynamic and increased spending on certain R&D programs, partially offset by favorable currency effects, net of hedging.
Fourth quarter 2019 R&D expenses were net of research tax credits in France and Italy, which amounted to $37 million, compared to $29 million in the third quarter of 2019 and $39 million in the fourth quarter of 2018.
Question:
Why did on sequential basis the operating expenses increased?
Answer: driven by seasonality and salary dynamic.